Enhanced Techniques for Traversing Ray Tracing Acceleration Structures

ABSTRACT

Enhanced techniques applicable to a ray tracing hardware accelerator for traversing a hierarchical acceleration structure are disclosed. For example, traversal efficiency is improved by combining programmable traversals based on ray operations with per-node static configurations that modify traversal behavior. The per-node static configurations enable creators of acceleration data structures to optimize for potential traversals without necessarily requiring detailed information about ray characteristics and ray operations used when traversing the acceleration structure. Moreover, by providing for selective exclusion of certain nodes using per-node static configurations, less memory is needed to express an acceleration structure that includes, for example, different geometric levels of details corresponding to a single object.

CROSS-REFERENCE TO RELATED PATENTS AND APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/829,954 filed Jun. 1, 2022, which is a continuation application of,and claims priority to, U.S. application Ser. No. 16/898,980 filed Jun.11, 2020, now U.S. Pat. No. 11,380,041 issued on Jul. 5, 2022 the entirecontents of which is herein incorporated by reference.

This application is related to the following commonly-assigned USpatents and patent applications, the entire contents of each of whichare incorporated by reference:

-   -   U.S. application Ser. No. 16/898,980 titled “Enhanced Techniques        for Traversing Ray Tracing Acceleration Structures” filed Jun.        11, 2020;    -   U.S. application Ser. No. 14/563,872 titled “Short Stack        Traversal of Tree Data Structures” filed Dec. 8, 2014;    -   U.S. Pat. No. 9,582,607 titled “Block-Based Bounding Volume        Hierarchy”;    -   U.S. Pat. No. 9,552,664 titled “Relative Encoding For A        Block-Based Bounding Volume Hierarchy”;    -   U.S. Pat. No. 9,569,559 titled “Beam Tracing”;    -   U.S. Pat. No. 10,025,879 titled “Tree Data Structures Based on a        Plurality of Local Coordinate Systems”;    -   U.S. application Ser. No. 14/737,343 titled “Block-Based        Lossless Compression of Geometric Data” filed Jun. 11, 2015;    -   U.S. patent application Ser. No. 16/101,066 titled Method for        Continued Bounding Volume Hierarchy Traversal on Intersection        Without Shader Intervention;    -   U.S. patent application Ser. No. 16/101,109 titled “Method for        Efficient Grouping of Cache Requests for Datapath Scheduling”;    -   U.S. patent application Ser. No. 16/101,247 titled “A Robust,        Efficient Multiprocessor-Coprocessor Interface”;    -   U.S. patent application Ser. No. 16/101,180 titled        “Query-Specific Behavioral Modification of Tree Traversal”;    -   U.S. patent application Ser. No. 16/101,148 titled “Conservative        Watertight Ray Triangle Intersection”;    -   U.S. patent application Ser. No. 16/101,196 titled “Method for        Handling Out-of-Order Opaque and Alpha Ray/Primitive        Intersections”; and    -   U.S. patent application Ser. No. 16/101,232 titled “Method for        Forward Progress and Programmable Timeouts of Tree Traversal        Mechanisms in Hardware”.

FIELD

The present technology relates to computer graphics, and moreparticularly to ray tracers. More particularly, the technology relatesto hardware acceleration of computer graphics processing including butnot limited to ray tracing. The example non-limiting technology hereinalso relates to efficient and flexible ray intersection tests thatprovide for selective execution of programmable ray operations.

BACKGROUND & SUMMARY

Real time computer graphics have advanced tremendously over the last 30years. With the development in the 1980's of powerful graphicsprocessing units (GPUs) providing 3D hardware graphics pipelines, itbecame possible to produce 3D graphical displays based on texture-mappedpolygon primitives in real time response to user input. Such real timegraphics processors were built upon a technology called scan conversionrasterization, which is a means of determining visibility from a singlepoint or perspective. Using this approach, three-dimensional objects aremodeled from surfaces constructed of geometric primitives, typicallypolygons such as triangles. The scan conversion process establishes andprojects primitive polygon vertices onto a view plane and fills in thepoints inside the edges of the primitives. See e.g., Foley, Van Dam,Hughes et al, Computer Graphics: Principles and Practice (2d Ed.Addison-Wesley 1995 & 3d Ed. Addison-Wesley 2014).

Hardware has long been used to determine how each polygon surface shouldbe shaded and texture-mapped and to rasterize the shaded, texture-mappedpolygon surfaces for display. Typical three-dimensional scenes are oftenconstructed from millions of polygons. Fast modern GPU hardware canefficiently process many millions of graphics primitives for eachdisplay frame (every 1/30^(th) or 1/60^(th) of a second) in real timeresponse to user input. The resulting graphical displays have been usedin a variety of real time graphical user interfaces including but notlimited to augmented reality, virtual reality, video games and medicalimaging. But traditionally, such interactive graphics hardware has notbeen able to accurately model and portray reflections and shadows.

There is another graphics technology which does perform physicallyrealistic visibility determinations for reflection and shadowing. It iscalled “ray tracing”. Ray tracing refers to casting a ray into a sceneand determining whether and where that ray intersects the scene'sgeometry. This basic ray tracing visibility test is the fundamentalprimitive underlying a variety of rendering algorithms and techniques incomputer graphics. Ray tracing was developed at the end of the 1960'sand was improved upon in the 1980's. See e.g., Appel, “Some Techniquesfor Shading Machine Renderings of Solids” (SJCC 1968) pp. 27-45;Whitted, “An Improved Illumination Model for Shaded Display” Pages343-349 Communications of the ACM Volume 23 Issue 6 (June 1980); andKajiya, “The Rendering Equation”, Computer Graphics (SIGGRAPH 1986Proceedings, Vol. 20, pp. 143-150). Since then, ray tracing has beenused in non-real time graphics applications such as design and filmmaking. Anyone who has seen “Finding Dory” (2016) or other Pixaranimated films has seen the result of the ray tracing approach tocomputer graphics—namely realistic shadows and reflections. See e.g.,Hery et al, “Towards Bidirectional Path Tracing at Pixar” (2016).

Generally, ray tracing is a rendering method in which rays are used todetermine the visibility of various elements in the scene. Ray tracingis used in a variety of rendering algorithms including for example pathtracing and Metropolis light transport. In an example algorithm, raytracing simulates the physics of light by modeling light transportthrough the scene to compute all global effects (including for examplereflections from shiny surfaces) using ray optics. In such uses of raytracing, an attempt may be made to trace each of many hundreds orthousands of light rays as they travel through the three-dimensionalscene from potentially multiple light sources to the viewpoint. Often,such rays are traced relative to the eye through the scene and testedagainst a database of all geometry in the scene. The rays can be tracedforward from lights to the eye, or backwards from the eye to the lights,or they can be traced to see if paths starting from the virtual cameraand starting at the eye have a clear line of sight. The testingdetermines either the nearest intersection (in order to determine whatis visible from the eye) or traces rays from the surface of an objecttoward a light source to determine if there is anything intervening thatwould block the transmission of light to that point in space. Becausethe rays are similar to the rays of light in reality, they makeavailable a number of realistic effects that are not possible using theraster based real time 3D graphics technology that has been implementedover the last thirty years. Because each illuminating ray from eachlight source within the scene is evaluated as it passes through eachobject in the scene, the resulting images can appear as if they werephotographed in reality. Accordingly, these ray tracing methods havelong been used in professional graphics applications such as design andfilm, where they have come to dominate over raster-based rendering.

Ray tracing can be used to determine if anything is visible along a ray(for example, testing for occluders between a shaded point on ageometric primitive and a point on a light source) and can also be usedto evaluate reflections (which may for example involve performing atraversal to determine the nearest visible surface along a line of sightso that software running on a streaming processor can evaluate amaterial shading function corresponding to what was hit—which in turncan launch one or more additional rays into the scene according to thematerial properties of the object that was intersected) to determine thelight returning along the ray back toward the eye. In classicalWhitted-style ray tracing, rays are shot from the viewpoint through thepixel grid into the scene, but other path traversals are possible.Typically, for each ray, the closest object is found. This intersectionpoint can then be determined to be illuminated or in shadow by shootinga ray from it to each light source in the scene and finding if anyobjects are in between. Opaque objects block the light, whereastransparent objects attenuate it. Other rays can be spawned from anintersection point. For example, if the intersecting surface is shiny orspecular, rays are generated in the reflection direction. The ray mayaccept the color of the first object intersected, which in turn has itsintersection point tested for shadows. This reflection process isrecursively repeated until a recursion limit is reached or the potentialcontribution of subsequent bounces falls below a threshold. Rays canalso be generated in the direction of refraction for transparent solidobjects, and again recursively evaluated. Ray tracing technology thusallows a graphics system to develop physically correct reflections andshadows that are not subject to the limitations and artifacts of scanconversion techniques.

Ray tracing has been used together with or as an alternative torasterization and z-buffering for sampling scene geometry. It can alsobe used as an alternative to (or in combination with) environmentmapping and shadow texturing for producing more realistic reflection,refraction and shadowing effects than can be achieved via texturingtechniques or other raster “hacks”. Ray tracing may also be used as thebasic technique to accurately simulate light transport inphysically-based rendering algorithms such as path tracing, photonmapping, Metropolis light transport, and other light transportalgorithms.

The main challenge with ray tracing has generally been speed. Raytracing requires the graphics system to compute and analyze, for eachframe, each of many millions of light rays impinging on (and potentiallyreflected by) each surface making up the scene. In the past, thisenormous amount of computation complexity was impossible to perform inreal time.

One reason modern GPU 3D graphics pipelines are so fast at renderingshaded, texture-mapped surfaces is that they use coherence efficiently.In conventional scan conversion, everything is assumed to be viewedthrough a common window in a common image plane and projected down to asingle vantage point. Each triangle or other primitive is sent throughthe graphics pipeline and covers some number of pixels. All relatedcomputations can be shared for all pixels rendered from that triangle.Rectangular tiles of pixels corresponding to coherent lines of sightpassing through the window may thus correspond to groups of threadsrunning in lock-step in the same streaming processor. All the pixelsfalling between the edges of the triangle are assumed to be the samematerial running the same shader and fetching adjacent groups of texelsfrom the same textures. In ray tracing, in contrast, rays may start orend at a common point (a light source, or a virtual camera lens) but asthey propagate through the scene and interact with different materials,they quickly diverge. For example, each ray performs a search to findthe closest object. Some caching and sharing of results can beperformed, but because each ray potentially can hit different objects,the kind of coherence that GPU's have traditionally taken advantage ofin connection with texture mapped, shaded triangles is not present(e.g., a common vantage point, window and image plane are not there forray tracing). This makes ray tracing much more computationallychallenging than other graphics approaches—and therefore much moredifficult to perform on an interactive basis.

In 2010, NVIDIA took advantage of the high degree of parallelism ofNVIDIA GPUs and other highly parallel architectures to develop theOptiX™ ray tracing engine. See Parker et al., “OptiX: A General PurposeRay Tracing Engine” (ACM Transactions on Graphics, Vol. 29, No. 4,Article 66, July 2010). In addition to improvements in API's(application programming interfaces), one of the advances provided byOptiX™ was improving the acceleration data structures used for findingan intersection between a ray and the scene geometry. Such accelerationdata structures are usually spatial or object hierarchies used by theray tracing traversal algorithm to efficiently search for primitivesthat potentially intersect a given ray. OptiX™ provides a number ofdifferent acceleration structure types that the application can choosefrom. Each acceleration structure in the node graph can be a differenttype, allowing combinations of high-quality static structures withdynamically updated ones.

The OptiX™ programmable ray tracing pipeline provided significantadvances, but was still generally unable by itself to provide real timeinteractive response to user input on relatively inexpensive computingplatforms for complex 3D scenes. Since then, NVIDIA has been developinghardware acceleration capabilities for ray tracing. See e.g., U.S. Pat.Nos. 9,582,607; 9,569,559; US 20160070820; US 20160070767; and the otherUS patents and patent applications cited above.

A basic task for most ray tracers is to test a ray against allprimitives (commonly triangles in one embodiment) in the scene andreport either the closest hit (according to distance measured along theray) or simply the first (not necessarily closest) hit encountered,depending upon use case. The naïve algorithm would be an O(n)brute-force search. However, due to the large number of primitives in a3D scene of arbitrary complexity, it usually is not efficient orfeasible for a ray tracer to test every geometric primitive in the scenefor an intersection with a given ray.

By pre-processing the scene geometry and building a suitableacceleration data structure in advance, however, it is possible toreduce the average-case complexity to O(log n). Acceleration datastructures, such as a bounding volume hierarchy or BVH, allow for quickdetermination as to which bounding volumes can be ignored, whichbounding volumes may contain intersected geometric primitives, and whichintersected geometric primitives matter for visualization and which donot. Using simple volumes such as boxes to contain more complex objectsprovides computational and memory efficiencies that help enable raytracing to proceed in real time.

FIGS. 1A-1C illustrate ray tracing intersection testing in the contextof a bounding volume 110 including geometric mesh 120. FIG. 1A shows aray 102 in a virtual space including bounding volumes 110 and 115. Todetermine whether the ray 102 intersects geometry in the mesh 120, eachgeometric primitive (e.g., triangle) could be directly tested againstthe ray 102. But to accelerate the process (since the object couldcontain many thousands of geometric primitives), the ray 102 is firsttested against the bounding volumes 110 and 115. If the ray 102 does notintersect a bounding volume, then it does not intersect any geometryinside of the bounding volume and all geometry inside the boundingvolume can be ignored for purposes of that ray. Because in FIG. 1A theray 102 misses bounding volume 110, any geometry of mesh 120 within thatbounding volume need not be tested for intersection. While boundingvolume 115 is intersected by the ray 102, bounding volume 115 does notcontain any geometry and so no further testing is required.

On the other hand, if a ray such as ray 104 shown in FIG. 1B intersectsa bounding volume 110 that contains geometry, then the ray may or maynot intersect the geometry inside of the bounding volume so furthertests need to be performed on the geometry itself to find possibleintersections. Because the rays 104, 106 in FIGS. 1B and 1C intersect abounding volume 110 that contains geometry, further tests need to beperformed to determine whether any (and which) of the primitives insideof the bounding volume are intersected. In FIG. 1B, further testing ofthe intersections with the primitives would indicate that even thoughthe ray 104 passes through the bounding volume 110, it does notintersect any of the geometry the bounding volume encloses(alternatively, as mentioned above, bounding volume 110 could be furthervolumetrically subdivided so that a bounding volume intersection testcould be used to reveal that the ray does not intersect any geometry ormore specifically which geometric primitives the ray may intersect).

FIG. 1C shows a situation in which the ray intersects bounding volume110 and contains geometry that ray 106 intersects. To perform real timeray tracing, an intersection tester tests each geometric primitivewithin the intersected bounding volume 110 to determine whether the rayintersects that geometric primitive.

The acceleration data structure most commonly used by modern ray tracersis a bounding volume hierarchy (BVH) comprising nested axis-alignedbounding boxes (AABBs). The leaf nodes of the BVH contain the primitives(e.g., triangles) to be tested for intersection. The BVH is most oftenrepresented by a graph or tree structure data representation. In raytracing, the time for finding the closest (or for shadows, any)intersection for a ray is typically order O(log n) for n objects whensuch an acceleration data structure is used. For example, AABB boundingvolume hierarchies (BVHs) of the type commonly used for modern raytracing acceleration data structures typically have an O(log n) searchbehavior.

The BVH acceleration data structure represents and/or references the 3Dmodel of an object or a scene in a manner that will help assist inquickly deciding which portion of the object a particular ray is likelyto intersect and quickly rejecting large portions of the scene the raywill not intersect. The BVH data structure represents a scene or objectwith a bounding volume and subdivides the bounding volume into smallerand smaller bounding volumes terminating in leaf nodes containinggeometric primitives. The bounding volumes are hierarchical, meaningthat the topmost level encloses the level below it, that level enclosesthe next level below it, and so on. In one embodiment, leaf nodes canpotentially overlap other leaf nodes in the bounding volume hierarchy.

NVIDIA's RTX platform includes a ray tracing technology that bringsreal-time, cinematic-quality rendering to content creators and gamedevelopers. See https://developer.nvidia.com/rtx/raytracing. In many ormost implementations including NVIDIA RT Cores, the bounding volumessuch as shown in FIG. 1A-1C use axis-aligned bounding boxes (“AABBs”),which can be compactly stored and easily tested for ray intersection. Ifa ray intersects against the bounding box of the geometry, then theunderlying geometry is then tested as well. If a ray does not intersectagainst the bounding box of the geometry though, then that underlyinggeometry does not need to be tested. As FIGS. 1A-1C show, a hierarchy ofAABB's is created to increase the culling effect of a single AABBbounding box test. This allows for efficient traversal and a quickreduction to the geometry of interest.

Nvidia's RTX platform introduced programmable ray operations that canchange traversal in hardware of an acceleration data structure in ahighly dynamic, query-specific manner. Using such ray operations, eachray query specifies test parameters, a test opcode and a mapping of testresults to actions. In an example ray tracing implementation, thedefault behavior of a ray traversing a bounding volume hierarchy ischanged in accordance with results of a test performed using the testopcode and test parameters specified in the ray data structure andanother test parameter specified in nodes of the acceleration datastructure. See e.g., US 2020/0051315.

The ray tracing API extensions for DirectX Raytracing (DXR) FunctionalSpecification v1.12 (Apr. 6, 2020) include an “Instance Masking” APIfeature for an acceleration data structure that, for example, enablescertain kinds of culling in the acceleration data structure. The DXRspecification provides for an instance mask to be specified for aninstance node in an acceleration structure, and for an instance nodeinclusion mask to be specified for a ray. During traversal of anacceleration structure with the ray, only those nodes in theacceleration structure that have an instance mask that has apredetermined value relative to the instance inclusion mask of the rayare further traversed and/or intersection tested. That is, the maskspecified in the ray is intended to match, according to a predeterminedlogical operation (e.g. AND), nodes that are to be included in thetraversal. Nvidia's RTX platform used the programmable ray operationsmentioned above to support the instance masking function in DXR.However, the RTX platform requires that, in order for the programmableray operations to be used for instance masking, all instance nodes inthe acceleration structure are configured with an instance mask that canbe logically compared with the instance inclusion mask in the ray.

The requirement, such as that described above, to have all nodes or allnodes of a certain type, configured in a certain way in order to havethe programmable ray operation apply to some nodes can reduce theflexibility of the programmable ray operations. Thus, furtherimprovements of the programmable ray operation capability are desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show example simplified ray tracing tests to determinewhether the ray passes through a bounding volume containing geometry andwhether the ray intersects geometry within the bounding volume.

FIG. 2A shows a part of an example acceleration data structure.

FIG. 2B shows a manner in which the example acceleration data structureof FIG. 2A can be modified to support certain programmable rayoperations at the expense of increased memory use.

FIG. 3 illustrates acceleration data structure such as that shown inFIG. 2A, when adapted, in accordance with some embodiments, in a mannerthat does not cause the increased memory use as in the acceleration datastructure shown in FIG. 2B.

FIGS. 4, 5A and 5B show example processes for per-node static behaviormodification of tree traversal in hardware, in accordance with someembodiments.

FIGS. 6A and 6B show example bounding volume hierarchy representations,according to some embodiments.

FIG. 7 illustrates an example non-limiting ray tracing graphics systemaccording to some embodiments.

FIG. 8 is a flowchart of an example non-limiting ray tracing graphicspipeline according to some embodiments.

FIG. 9 is a flowchart of example non-limiting hardware based ray tracingoperations, according to some embodiments.

FIG. 10 shows a simplified example non-limiting traversal co-processorcomprising a tree traversal unit (TTU), according to some embodiments.

FIG. 11 is a flowcharts of example TTU processing, according to someembodiments.

FIGS. 12A and 12B illustrate more detailed ray tracing pipelines,according to some embodiments.

FIG. 13 shows an example traversal stack for use by the ray tracingpipeline, according to some embodiments.

FIG. 14 shows an example results queue for use by the ray tracingpipeline according to some embodiments.

FIG. 15A shows an example ray query structure, according to someembodiments.

FIGS. 15B and 15C show tables of values for certain ray opcodes and rayparameters for use in programmable ray operations, according to someembodiments.

FIGS. 16A, 16B, and 16C show a data structure format of an example nodestructure, according to some embodiments.

FIG. 17 shows an example process with a per-node selectable programmableray operation in hardware, according to some embodiments.

FIG. 18 is a flowchart of an example process to generate an image,according to some embodiments.

DETAILED DESCRIPTION OF NON-LIMITING EMBODIMENTS

The already highly efficient ray tracing capabilities of the Nvidia RTXtechnology, which provides hardware-accelerated testing of rays againstthe hierarchically-arranged bounding boxes that encompass objectgeometries in a scene and also the underlying object geometries as well,can be improved even more by, for example, further improving theflexibility and efficiency of the programmable ray operation.

One aspect of further improving the programmable ray operationcapability is made by providing for the instance masking capability,which was previously tied to the programmable ray operation, to beseparated from the programmable ray operation, thereby freeing theprogrammable ray operation capability for implementing other functions.The application U.S. Ser. No. 16/897,909 describes ray traversal with acombination of instance masking and programmable ray operation. Inanother improvement of the capabilities of the RTX platform, thedeveloper is provided with the capability to rebuild parts of the sceneon a frame-by-frame basis in that the same top level accelerationstructure can be reused with relatively small modifications for changingscenes in order to reduce false positives among detected ray-boundingvolume intersections in a “top-level instancing” capability described inapplication U.S. Ser. No. 16/897,745.

Whereas new capabilities such as but not limited to those described inthe paragraph above enable a more flexible programmable ray operationand also provides developers with more flexibility with fine tuning theacceleration structures to particular scenes and user views, theinventors realized that the programmable ray operation can be evenfurther improved by reducing the requirements for the accelerationstructure to be specifically tailored to particular rays or ray typesand also by allowing for some nodes to be traversed without beingsubjected to the programmable ray operation. By reducing therequirements for the acceleration structure to be specifically tailoredto particular rays or ray types, in other words, by lessening the amountof coordination that must occur between the developer of theacceleration data structure and the programmers implementing the rayoperations, more flexible and possibly more efficient use of bothacceleration structures and rays is enabled. By allowing certain nodesin the acceleration data structure to be traversed without beingsubjected to the programmable ray operation the memory requirements forthe acceleration data structure are reduced. Example embodiments of thisdisclosure lessen the amount of coordination that must occur between thedeveloper of the acceleration data structure and the programmersimplementing the ray operations, and also allow certain nodes in theacceleration data structure to be traversed without being subjected tothe programmable ray operation.

Certain example embodiments of this disclosure provide a ray tracingcoprocessor hardware device that enables hardware-accelerated raytracing in which respective nodes of the acceleration data structure canbe configured to selectively ignore the programmable ray operation, i.e.the example embodiments allow for respective nodes in the accelerationdata structure to include a “force ray always” (FRA) flag which isconsidered by the coprocessor in order to either ignore or perform theprogrammable ray operation accordingly. The programmable ray operationmay be specified in an opcode included in the ray. In someimplementations, the programmable opcode-based ray operation can beimplemented as described in U.S. patent application Ser. No. 16/101,180titled “Query-Specific Behavioral Modification of Tree Traversal”, whichis hereby incorporated by reference in its entirety.

FIG. 2A illustrates a portion of an example acceleration data structure200. As described in more detail below in relation to construction ofBVHs, the acceleration data structure 200 includes top-levelacceleration structure (TLAS) 202 and a bottom-level accelerationstructure (BLAS) 204. Although a scene can typically be represented byone TLAS tree, the BLAS for the scene may comprise many separateacceleration structures each encompassing one or more objects in thescene. Each separate BLAS tree, for example, may encompass one or moreobjects in a particular coordinate space, and each such BLAS tree may beconnected to the TLAS by an instance node which specifies a transformfrom the world coordinate space in which the scene and the TLAS isdefined to the object coordinate space of that respective object. In theexample shown in FIG. 2A, the BLAS for the scene incorporates anacceleration structure representing the geometry of a car (car geometry206) and another acceleration structure representing the geometry of asimple object such as a box (simple box geometry 208). Some objects,typically objects that have a complex structure or design, may haveseveral models, each defined in a different level of detail (LOD). Othersimpler objects, may have only a single model. For example, the cargeometry 206 has three separate models defining the same car—an LOD 1model 216 using about 10000 triangles, an LOD 2 model 218 using about1000 triangles, and an LOD 3 model 220 using about 50 triangles. Thesemodels of the car are arranged in a BLAS tree rooted at node 212. Thesimple box is defined by a model 222 as the only model available forthat simple box, which is arranged in a BLAS tree rooted at 214. Whenthe acceleration data structure for the scene is constructed, the BLAStree 206 and the BLAS tree 208 are connected by their respective rootnodes 212 and 214 to the TLAS rooted at node 210.

Ray traversing such as that provided by Nvidia's RTX platform, providesfor programmable ray operations that allow each ray to, among otherthings, select the LOD of an object that it tests for intersections.However, the ray traversing implementation of the RTX platform, requiresthat, if a programmable ray operation were to apply to one BLAS, everyBLAS in that acceleration structure also supports that programmable rayoperation. That is, for example, if a programmable ray operation is usedto select the LOD in one BLAS, then every BLAS in that acceleration mustbe configured to include multiple LOD and permit that programmable rayoperation to be performed. If only some BLAS in an acceleration datastructure have models of an object or objects in multiple LOD, whileother BLAS in the acceleration structure do not include models inmultiple LOD, then the programmable ray operation for selecting the LODcannot be used on that acceleration structure on the RTX platform. Forexample, on the RTX platform, the ray operation to select the LOD cannotbe optimally performed for the acceleration structure of FIG. 2A. As canbe seen in FIG. 2A, whereas multiple models for different LOD areavailable in the acceleration structure 200 for the car geometry 206,only a single model is available for the simple box 208. On the RTXplatform, the programmable ray operation for selecting LOD cannot beoptimally performed on acceleration data structures such as theacceleration data structure 200.

For the reasons described in the above paragraph, and for other reasonstoo, various inefficiencies are sometimes introduced in order to improvethe efficiency and/or flexibility of the programmable ray operationcapabilities on the RTX platform. For example, in order to preserve thecapability of performing the programmable ray operation in many types ofscenes, sometimes numerous nodes or entire portions of acceleration datastructures that are likely to be unused (e.g. not be traversed by a ray)are added to the acceleration data structure for a scene. FIG. 2Billustrates an example.

The acceleration data structure 230 shown in FIG. 2B may be identical tothe acceleration data structure 200 shown in FIG. 2A except for thearrangement of the simple box geometry 208 of the acceleration datastructure 200. That is, the TLAS 202 in acceleration structure 230 maybe identical to TLAS 202 in the acceleration structure 200, and the BLAS204′ may be identical to BLAS 204 except for the arrangement of thesimple box geometry 208. The tree for the simple box geometry 208 now,represented in FIG. 2B as simple box geometry 232, includes a new rootnode 234 introduced as the parent of the original simple box geometrytree rooted at node 214, which was the root of the simple box geometryBLAS tree in FIG. 2A. As adapted, the new simple box geometry BLAS rootnode 234 has two child nodes—the tree rooted at the original BLAS rootnode 214 and the root 214′ of a duplicate of the tree rooted at 214. Thenodes 214′ and 222′ are identical to nodes 214 and 222 of the originalBLAS tree, except that one of the nodes 214 and 214′ has its rayoperation control parameters inverted while the other does not (e.g. onehas the invert bit set for the “rval”, and the other does not; rval andinvert bit are described in relation to FIG. 17 ).

Since the corresponding root node of the BLAS now include multiple childnodes, the programmable ray operation for selecting an LOD can beperformed for the simple box geometry 208 too. When the programmable rayoperation is performed on the nodes 214 and 214′, since one has theinverse of the ray operation control parameter stored in the other, theray operation succeeds only on one of the nodes 214 and 214′, and thusit is guaranteed that the ray traverses only one of the trees. In thismanner, even if the BLAS as provided by the developer may not includemodels of multiple LODs for some of the objects in the scene, aprogrammable ray operation for dynamically selecting the mostappropriate LOD for one or more rays can be used on the accelerationstructure 200′.

However, the newly introduced nodes—the duplicated nodes 214′ and 222′and the new BLAS root 234—introduce memory bloat 236 to the accelerationdata structure 200. The newly added portion 236 merely leads to the samemodel 222, or a duplicate 222′ of the model 222. While only the memorybloat associated with one BLAS tree, which also happens to be a smalltree, is shown in FIG. 2B, it is easily understood that the amount ofmemory bloat can be much larger. For example, if each BLAS tree is to beduplicated, the additional memory bloat would be at least twice the sizeof all the BLAS trees because of the node to node duplication of theBLAS trees and the addition of a new root node for each BLAS. Inaddition to the memory bloat, this technique of duplicating a part ofthe acceleration data structure may also add inefficiencies to thetraversal because two rays that otherwise might share cache lines withinan “always” pass BVH have to load independent cache lines instead.

FIG. 3 illustrates a portion of an acceleration data structure 300configured according to some embodiments to provide for efficient andflexible programmable ray operations while also avoiding problems suchas, for example, the memory bloat described in relation to FIG. 2B. Moreparticularly, acceleration data structure 300 can be obtained byadapting the acceleration data structure 200 to include support forper-node static traversal behavior modification according to someembodiments. The acceleration data structure 300 may be identical to theacceleration data structure 200, with the exception that in accelerationdata structure 300 some or all of the nodes include a “force ray always”(FRA) flag of which the value can be set in each node to ensure that thenode, or in some embodiments, the node's child nodes, ignore the rayoperation.

More specifically, the TLAS 302 rooted at node 310 may be identical instructure to the TLAS 202 rooted at node 210 and shown in FIG. 2A, andBLAS 304 may be identical in structure to the BLAS 204 shown in FIG. 2A.However, in contrast to the acceleration data structure of FIG. 2A, atleast some nodes in acceleration data structure 300 are configured withthe FRA flag. For example, FIG. 3 shows the root nodes 312 and 314 ofeach of the BLAS (306 and 308) are configured with the FRA flag inrelation to a programmable ray operation. In the illustrated example,node 312 which connects the car geometry 306 to the TLAS is set withFRA=0 (FRA flag is not set) thereby indicating that the programmable rayoperation is to be applied at node 312, and node 314 which connects thecar geometry 308 to the TLAS is set with FRA=1 (FRA flag is set) therebyindicating that the programmable ray operation is to be ignored at node314. When set, the FRA flag can be considered to make the complet or thenode as “always” pass to ignore the programmable ray operation specifiedin the ray. Thus, when a ray which includes a ray operation, such as,for example, the Tmin ray operation (e.g. the “TMIN_LESS” ray operationdescribed in relation to FIG. 17 ) that selects a particular LODaccording to the t-distance of (e.g. the length of) the ray, is used totraverse the acceleration data structure 300, the ray operation will beperformed at node 312 (because FRA=0), but will not be performed at node314 (because FRA=1). This has the effect of allowing a ray with theprogrammable ray operation Tmin to traverse the acceleration datastructure 300 in order to yield the desired selecting of the mostappropriate LOD based on the length of the ray only when multiple modelsof different LOD are available for an object or group of objects,without requiring the addition of duplicated nodes and/or trees to theacceleration data structure causing memory bloat.

FIGS. 4, 5A and 5B show example processes for per-node static behaviormodification of tree traversal in hardware, in accordance with someembodiments. FIG. 4 illustrates a process 400 for traversing anacceleration data structure by a ray in order to obtain intersectioninformation for the ray, with the acceleration data structure beingconfigured for per-node static behavior modification of the treetraversal. FIG. 5A illustrates more details of the effect on process 400when the FRA flag has been set. FIG. 5B illustrates more details of theprocess 400 when the FRA flag specifies more than a binary decision(e.g., true/false or set/not set) as in FIG. 5A.

Process 400 may be performed in a real time ray tracing graphics system700 (see FIG. 7 ) by the traversal coprocessor 738. Example componentsof the traversal coprocessor 738 according to some embodiments are shownin FIG. 10 . A general description of ray traversal processing on thetraversal coprocessor 738 is provided in relation to FIGS. 8-9, 11, 12Aand 12B. Process 400 relates to certain aspects of the traversalprocessing in coprocessor 738 that involves the FRA flag and theprogrammable ray operation.

At operation 402, the traversal coprocessor 738 receives a ray queryfrom the streaming multiprocessor (SM) 732. The ray query includes rayinformation for the ray, and acceleration structure information for theacceleration structure or the portion thereof to be traversed by theray. The ray information includes ray operation information for aprogrammable ray operation, including, for example, an opcode definingthe ray operation, and one or more parameters to be used in the rayoperation. An example ray query data structure is shown in FIG. 15A,examples of opcodes are shown in FIG. 15B, and example ray parametersare shown in FIG. 15C.

At operation 404, the traversal coprocessor 738 accesses theacceleration structure using the acceleration structure informationincluded in the received ray query. The acceleration structure may haveone or more nodes with configurations that can be used for per-nodestatic modification of tree traversal behavior as it pertains to thetest for the programmable ray operation. For example, one or more nodesof the acceleration structure is configured with a FRA flag indicating,when the FRA flag is set, that the programmable ray operation should notbe applied at a particular node

The acceleration structure shown in FIG. 3 may be an accelerationstructure, or portion thereof, traversed according to process 400. Asdescribed above, in the acceleration structure of FIG. 3 , at least thenode 314 includes an FRA flag that is set (e.g. value of 1). In the sameFIG. 3 , node 312 is shown with the FRA corresponding to not being set(e.g. value of 0). Example node data structures each having a FRA flagand ray operation parameters are shown in FIGS. 16A and 16B.

At operation 406, the acceleration structure is traversed with the rayspecified in the received ray query. During the traversal, a node whichincludes the FRA flag is encountered. In some embodiments, all nodes inthe acceleration structure include the FRA flag indicating whether theprogrammable ray operation test is applicable, while in some otherembodiments only certain nodes (e.g., predetermined one or more types ofnodes and/or a flag indicating validity of an FRA flag in a node)include the FRA flag. For example, the acceleration structure 300 mayinclude a valid FRA flag in every node in some embodiments, or, in someother embodiments, may have valid FRA flags only in the root nodes ofthe BLAS structures. FIG. 3 shows the FLA flag being configured ininstance nodes 312 and 314 which are the roots of example BLASstructures 306 and 308 respectively.

At operation 406, if the encountered node has a valid FRA flag with avalue of 0 indicating that the FRA flag is not set, the programmable rayoperation test according to the opcode (and optional ray parameters)specified in the ray and one or more values that are specified in thenode is performed at the encountered node. Example programmable rayoperations, referred to below as “RayOp”, are described in relation toFIGS. 15B and 17 . According to some embodiments, when the node has anFRA flag set to 0, then the ray's programmable ray operation isperformed for each of that node's child nodes.

If, on the other hand, the FRA flag in the encountered node has a valueof 1 indicating that the FRA flag is set, then the programmable rayoperation specified in the opcode is not performed at the node for anyof its child nodes thus ignoring the ray operation for the encounterednode. The capability of ignoring the programmable ray operation only forsome nodes, as introduced by the FRA flag, enables the utilization ofthe powerful programmable ray operations with a wide variety of scenesand scene models without incurring the memory bloat that was describedabove in relation to FIG. 3B.

If at operation 406, either based on the FRA flag only or based on thecombination of the FRA flag and the programmable ray operation, it isdetermined that the node's child nodes are to be traversed, then atoperation 408, process 400 continues the traversal of the subtree(s)rooted at the node and subsequently returns whether or not the rayintersected objects in the scene.

It should be noted that whereas the FRA flag determines whether or not aprogrammable ray operation, such as a ray operation defined at least inpart by an opcode included in the ray, is performed at a node, the FRAflag may not have any impact on other operations performed on the nodesuch as, for example, transforming the ray from one coordinate space toanother as specified in the node (e.g. as specified in instance nodes),instance masking that is independent of the programmable ray operation,etc. Non-limiting examples of traversal behavior that are modified byusing the FRA flag in an encountered node to control whether or not aray's programmable ray operation is performed at the encountered nodemay include culling behavior, dynamic selection of nodes to traversebased on ray characteristics, selection of which of the child nodes ofthe encountered are traversed, order in which child nodes of theencountered node are traversed, returning partial or intermediatetraversal results from the hardware coprocessor to the processor, etc.

FIG. 5A illustrates a process 500 for performing step 406, according tosome embodiments. Process 500 may be invoked when, in process 400, thetraversal enters a node.

At step 502 it is determined whether the FRA flag is set in the node. Inthis embodiment, the FRA flag may be a binary flag with value 1indicating that the flag is set, and value 0 indicating that the flag isnot set. It will be understood that alternative embodiments mayrepresent the set flag with 0 and with 1 otherwise.

If the FRA flag is set at the node, it is an indication that theprogrammable ray operation should not apply at that node. Therefore, ifat step 502, it is determined that the FRA flag is set, process 500proceeds to continue traversal at step 508.

If it is determined that the FRA flag is not set at the node, then it isan indication that the programmable ray operation is not overridden forthe node, and at step 504, the programmable ray operation is performed.As noted above, the programmable ray operation is described below inrelation to FIG. 17 .

At step 506, in accordance with a result of the programmable rayoperation performed at step 504, the culling of a node/subtree may beperformed. The result of the ray operation may be an action other thanculling, such as, for example, determining a particular ordering inwhich a set of child nodes of the node are to be traversed.

At step 508, the traversal continues in accordance with the action takenat step 506 or in accordance with the decision made at step 502 toentirely skip or ignore the programmable ray operation with respect tothis node.

FIG. 5B illustrates another process 500′ which may be used in performingstep 406, according to another embodiment. Process 500′ may beperformed, when the FRA flag is more than a binary flag, and inparticular, when the FRA flag is two or more bits wide and encodes twoor more different actions that can be performed.

The processing steps of process 500′ may be similar to those of process500 described above, with the exception that the FRA flag is, instead ofbeing a binary flag, is multi-bit, and is configured to specify adifferent action for each of two or more respective values of the flag.For example, with a FRA flag that is two bits wide, a value of 0 may bedefined as “do not ignore programmable ray operation” (i.e. proceed toperform the programmable ray operation specified by the ray); a value of1 may be defined as “ignore ray operation—proceed to traversal” (i.e. donot perform the programmable ray operation), and the values 2 and 3 maybe defined for performing a predetermined first operation and apredetermined second operation respectively, where the predeterminedfirst and second operations may be any operation such as, for example,selecting a child node according to a predetermined criteria (e.g. LOD,first occurring child node according to a predetermined ordering, valueof one or more fields in a child node, etc.) for culling or traversal.

Thus, when at step 502, it is determined that the flag is set, at step505, the action corresponding to the value of the FRA flag is performed.After step 505, the processing proceeds to continuing traversal at step508. The processing logic of steps 502, 504, 506 and 508 may beidentical in processes 500 and 500′.

Further description of traversal of the accelerated data structure,performed on the traversal coprocessor, based on the ray information andacceleration structure information provided at step 402 is described inrelation to FIGS. 12A-B. The ray intersection information returned fromthe traversal coprocessor is used for rendering the scene. The renderingof the scene using the intersection information is described below (e.g.step 1858) in relation to the example process of generating an imageshown in FIG. 18 .

The descriptions of the process for the per-node static programmable rayoperation in relation to FIGS. 3, 4, 5A-5B and also the description inrelation to FIG. 17 below, are examples. However, embodiments are notlimited to programmable ray operations that select geometric LOD.

Building a Bounding Volume Hierarchy

As described above, an acceleration data structure comprises a hierarchyof bounding volumes (bounding volume hierarchy or BVH) that recursivelyencapsulates smaller and smaller bounding volume subdivisions. Thelargest volumetric bounding volume may be termed a “root node.” Thesmallest subdivisions of such hierarchy of bounding volumes (“leafnodes”) contain items. The items could be primitives (e.g., polygonssuch as triangles) that define surfaces of the object. Or, an item couldbe a sphere that contains a whole new level of the world that exists asan item because it has not been added to the BVH (think of the collarcharm on the cat from “Men in Black” which contained an entire miniaturegalaxy inside of it). If the item comprises primitives, the traversalco-processor upon reaching an intersecting leaf node tests rays againstthe primitives associated with the leaf node to determine which objectsurfaces the rays intersect and which object surfaces are visible alongthe ray.

Building a BVH can occur in two parts: static and dynamic. In manyapplications, a complex scene is preprocessed and the BVH is createdbased on static geometry of the scene. Then, using interactive graphicsgeneration including dynamically created and manipulated moving objects,another part of the BVH (or an additional, linked BVH(es) can be builtin real time (e.g., in each frame) by driver or other software runningon the real time interactive graphics system. BVH construction need notbe hardware accelerated (although it may be in some non-limitingembodiments) but may implemented using highly-optimized softwareroutines running on streaming multiprocessors (SMs) (e.g. SM 732) and/orCPU (e.g. CPU 120) and/or other development systems e.g., duringdevelopment of an application.

The first stage in BVH acceleration structure construction acquires thebounding boxes of the referenced geometry. This is achieved by executingfor each geometric primitive in an object a bounding box procedure thatreturns a conservative axis-aligned bounding box (AABB) for its inputprimitive. Aligning bounding boxes with the axes of the relevantcoordinate systems for the geometry provides for increased efficiency ofreal time geometrical operations such as intersection testing andcoordinate transforms as compared for example to oriented bounding boxes(OBB's), bounding spheres, or other approaches. However, those skilledin the art will understand that the example non-limiting approachesherein can also be applied to more expensive bounding constructs such asOBBs, bounding spheres and other bounding volume technology.

Already subdivided bounding volumes that do include at least one portionof the geometry in a scene can be still further recursivelysubdivided—like the emergence of each of a succession of littler andlittler cats from the hats of Dr. Seuss's' The Cat In The Hat Comes Back(1958). The number and configurations of recursive subdivisions willdepend on the complexity and configuration of the 3D object beingmodeled as well as other factors such as desired resolution, distance ofthe object from the viewpoint, etc. One example subdivision scheme is aso-called 8-ary subdivision or “octree” in which each volume issubdivided into eight smaller volumes of uniform size, but many otherspatial hierarchies and subdivision schemes are known such as a binarytree, a four-ary tree, a k-d tree, a binary space partitioning (BSP)tree, and a bounding volume hierarchy (BVH) tree. See e.g., U.S. Pat.No. 9,582,607.

At some level of subdivision (which can be different levels fordifferent parts of the BVH), the BVH construction process encountersgeometry making up the encapsulated object being modeled. Using theanalogy of a tree, the successive volumetric subdivisions are the trunk,branches, boughs and twigs, and the geometric is finally revealed at thevery tips of the tree, namely the leaves. At this point, the BVHconstruction process for example non-limiting embodiments hereinperforms an optimization at this stage to spot, using heuristic or otheranalytical techniques (which might include artificial intelligenceand/or neural networks in some embodiments), those leaf nodes thatpresent poor fits with respect to the geometry they contain.

This process continues until all bounding volumes containing geometryhave been sufficiently subdivided to provide a reasonable number ofgeometric primitives per bounding box. The real time ray tracer thatuses the BVH will determining ray-primitive intersections by comparingthe spatial xyz coordinates of the vertices of each primitive with thexyz coordinates of the ray to determine whether the ray and the surfacethe primitive defines occupy the same space. The ray-primitiveintersection test can be computationally intensive because there may bemany triangles to test. In many cases, it may be more efficient tofurther volumetrically subdivide and thereby limit the number ofprimitives in any “leaf node” to something like 16 or fewer.

The resulting compressed tree comprising compressed treelets(“complets”) is written out into a data structure in memory for lateruse by the graphics processing hardware/software during e.g., real timegraphics processing that includes real time ray tracing.

FIGS. 6A and 6B show a recursively-subdivided bounding volume of a 3Dscene (FIG. 6A) and a corresponding tree data structure (FIG. 6B) thatmay be accessed by the ray tracer and used for hardware-acceleratedoperations. The tree data structure may be stored in memory andretrieved on demand based on queries.

The division of the bounding volumes may be represented in ahierarchical tree data structure with the large bounding volumerepresented by a parent node of the tree and the smaller boundingvolumes represented by children nodes of the tree that are contained bythe parent node. The smallest bounding volumes are represented as leafnodes in the tree and identify one or more geometric primitivescontained within these smallest bounding volumes.

The tree data structure includes a plurality of nodes arranged in ahierarchy. The root nodes N1 of the tree structure correspond tobounding volume N1 enclosing all of the primitives O1-O8. The root nodeN1 may identify the vertices of the bounding volume N1 and childrennodes of the root node.

In FIG. 6A, bounding volume N1 is subdivided into bounding volumes N2and N3. Children nodes N2 and N3 of the tree structure of FIG. 6Bcorrespond to and represent the bounding volumes N2 and N3 shown in FIG.6A. The children nodes N2 and N3 in the tree data structure identify thevertices of respective bounding volumes N2 and N3 in space. Each of thebounding volumes N2 and N3 is further subdivided in this particularexample. Bounding volume N2 is subdivided into contained boundingvolumes N4 and N5. Bounding volume N3 is subdivided into containedbounding volumes N6 and N7. Bounding volume N7 include two boundingvolumes N8 and N9. Bounding volume N8 includes the triangles O7 and O8,and bounding volume N9 includes leaf bounding volumes N10 and N11 as itschild bounding volumes. Leaf bounding volume N10 includes a primitiverange (e.g., triangle range) O10 and leaf bounding volume N11 includesan item range O9. Respective children nodes N4, N5, N6, N8, N10 and N11of the FIG. 6B tree structure correspond to and represent the FIG. 6Abounding volumes N4, N5, N6, N8, N10 and N11 in space.

The FIG. 6B tree in this particular example is only three to six levelsdeep so that volumes N4, N5, N6, N8, N10 and N11 constitute “leafnodes”—that is, nodes in the tree that have no child nodes. FIG. 6Ashows that leaf node bounding volumes N4, N6, and N8 each contains twotriangles of the geometry in the scene. For example, volumetricsubdivision N4 contains triangles O1 & O2; volumetric subdivision N6contains trials O5 & O6; and volumetric subdivision N8 containstriangles O7 & O8. FIG. 6A further shows that leaf node bounding volumeN5 contains a single cylinder O3 such as shown in that does not providea good fit for the AABB bounding volume N5 shown in dotted lines.Accordingly, in an example non-limiting embodiment herein, instead ofusing the larger AABB bounding volume N5 for the ray-bounding volumeintersection test, TTU 738 instead tests the ray against a plurality ofsmaller AABB bounding volumes that are arranged, positioned, dimensionedand oriented to more closely fit cylinder O3.

The tree structure shown in FIG. 6B represents these leaf nodes N4, N5,N6, and N7 by associating them with the appropriate ones of primitiveO1-O8 of the scene geometry. To access this scene geometry, the TTU 738traverses the tree data structure of FIG. 6B down to the leaf nodes. Ingeneral, different parts of the tree can and will have different depthsand contain different numbers of primitives. Leaf nodes associated withvolumetric subdivisions that contain no geometry need not be explicitlyrepresented in the tree data structure (i.e., the tree is “trimmed”).

According to some embodiments, the subtree rooted at N7 may represent aset of bounding volumes or BVH that is defined in a different coordinatespace than the bounding volumes corresponding to nodes N1-N3. Whenbounding volume N7 is in a different coordinate space from its parentbounding volume N3, an instance node N7′ which provides the raytransformation necessary to traverse the subtree rooted at N7, mayconnect the rest of the tree to the subtree rooted at N7. Instance nodeN7′ connects the bounding volume or BVH corresponding to nodes N1-N3,with the bounding volumes or BVH corresponding to nodes N7 etc. bydefining the transformation from the coordinate space of N1-N3 (e.g.,world space, world coordinate space) to the coordinate space of N7 etc.(e.g., object space, object coordinate space).

In some embodiments, the tree or subtree rooted at N1 is associated witha parent node N1′ that is an instance node. Instance node N1′ maycontain, or may be associated with a transform for transforming a rayfrom a one coordinate space to another coordinate space. In someembodiments, N1′ may specify a transform from the world space to analternative world space and may be referred to as a “top level instancenode” such as that described in above mentioned application U.S. Ser.No. 16/897,745.

In more detail, seehttps://developer.nvidia.com/rtx/raytracing/dxr/DX12-Raytracing-tutorial-Part-1which describes top (TLAS) and bottom (BLAS) levels of an accelerationdata structure and ways to create a BVH using them. In one exampleimplementation herein, for each object or set of objects, a BLASbounding volume may be defined around the object(s)—and in the case ofmoving geometry, multiple bounding volumes may be defined for differenttime instants. That bounding volume(s) is in object space and canclosely fit the object(s). The resulting BLAS contains the fulldefinition of the geometry, organized in a way suitable for efficientlyfinding ray intersections with that geometry.

The BLAS is defined in object space. When creating a BVH, all of thoseindividual objects (each of which are in their own respective objectspaces) and associated subtreelets are placed into world space usingtransforms. The BVH thus specifies, for each BLAS subtree, transformsfrom object space to world space. Shaders use those transforms totranslate/rotate/scale each object into the 3D scene in world space.

The BVH meanwhile defines the TLAS bounding volumes in world space. TheTLAS can be thought of as an acceleration data structure above anacceleration data structure. The top TLAS level thus enables boundingvolumes and ray-complet tests, and in one embodiment needs no transformsbecause the ray is specified in world space. However, in the examplenon-limiting embodiment herein, the TLAS bounding volumes for objectsunder motion may also be temporally-encoded with multiple spatialpositions to allow hardware circuitry to calculate a particular spatialposition at the instant of a ray for purposes of ray-bounding volumeintersection testing.

As the ray tracing system traverses downward to a certain point in thetree and encounters an instance node, the mode switches from TLAS (inworld space) to BLAS (in object space). The object vertices are in oneembodiment defined in object space as are the BLAS bounding volumes(which can be different from the TLAS bounding volumes). The transforminformation in the complet is used to transform the ray from world spaceinto object space to test against the BLAS subtree. In one embodiment,the same interpolation hardware used for TLAS ray-bounding volumeintersection testing can also be used for BLAS ray-bounding volumeintersection testing—and different (e.g., higher precision) hardware maybe provided for vertex interpolation and ray-primitive intersectiontesting on the BLAS level.

The acceleration structure constructed as described above can be used toadvantage by software based graphics pipeline processes running on aconventional general purpose computer. However, the presently disclosednon-limiting embodiments advantageously implement the above-describedtechniques in the context of a hardware-based graphics processing unitincluding a high performance processors such as one or more streamingmultiprocessors (“SMs”) and one or more traversal co-processors or “treetraversal units” (“TTUs”)—subunits of one or a group of streamingmultiprocessor SMs of a 3D graphics processing pipeline. The followingdescribes the overall structure and operation of such as systemincluding a TTU 738 that accelerates certain processes supportinginteractive ray tracing including ray-bounding volume intersectiontests, ray-primitive intersection tests and ray “instance” transformsfor real time ray tracing and other applications.

Example System Block Diagram

FIG. 7 illustrates an example real time ray interactive tracing graphicssystem 700 for generating images using three dimensional (3D) data of ascene or object(s) including the acceleration data structure constructedas described above.

System 700 includes an input device 710, a processor(s) 720, a graphicsprocessing unit(s) (GPU(s)) 730, memory 740, and a display(s) 750. Thesystem shown in FIG. 7 can take on any form factor including but notlimited to a personal computer, a smart phone or other smart device, avideo game system, a wearable virtual or augmented reality system, acloud-based computing system, a vehicle-mounted graphics system, asystem-on-a-chip (SoC), etc.

The processor 720 may be a multicore central processing unit (CPU)operable to execute an application in real time interactive response toinput device 710, the output of which includes images for display ondisplay 750. Display 750 may be any kind of display such as a stationarydisplay, a head mounted display such as display glasses or goggles,other types of wearable displays, a handheld display, a vehicle mounteddisplay, etc. For example, the processor 720 may execute an applicationbased on inputs received from the input device 710 (e.g., a joystick, aninertial sensor, an ambient light sensor, etc.) and instruct the GPU 730to generate images showing application progress for display on thedisplay 750.

Based on execution of the application on processor 720, the processormay issue instructions for the GPU 730 to generate images using 3D datastored in memory 740. The GPU 730 includes specialized hardware foraccelerating the generation of images in real time. For example, the GPU730 is able to process information for thousands or millions of graphicsprimitives (polygons) in real time due to the GPU's ability to performrepetitive and highly-parallel specialized computing tasks such aspolygon scan conversion much faster than conventional software-drivenCPUs. For example, unlike the processor 720, which may have multiplecores with lots of cache memory that can handle a few software threadsat a time, the GPU 730 may include hundreds or thousands of processingcores or “streaming multiprocessors” (SMs) 732 running in parallel.

In one example embodiment, the GPU 730 includes a plurality ofprogrammable high performance processors that can be referred to as“streaming multiprocessors” (“SMs”) 732, and a hardware-based graphicspipeline including a graphics primitive engine 734 and a raster engine736. These components of the GPU 730 are configured to perform real-timeimage rendering using a technique called “scan conversion rasterization”to display three-dimensional scenes on a two-dimensional display 750. Inrasterization, geometric building blocks (e.g., points, lines,triangles, quads, meshes, etc.) of a 3D scene are mapped to pixels ofthe display (often via a frame buffer memory).

The GPU 730 converts the geometric building blocks (i.e., polygonprimitives such as triangles) of the 3D model into pixels of the 2Dimage and assigns an initial color value for each pixel. The graphicspipeline may apply shading, transparency, texture and/or color effectsto portions of the image by defining or adjusting the color values ofthe pixels. The final pixel values may be anti-aliased, filtered andprovided to the display 750 for display. Many software and hardwareadvances over the years have improved subjective image quality usingrasterization techniques at frame rates needed for real-time graphics(i.e., 30 to 60 frames per second) at high display resolutions such as4096 x 2160 pixels or more on one or multiple displays 750.

To enable the GPU 730 to perform ray tracing in real time in anefficient manner, the GPU provides one or more “TTUs” 738 coupled to oneor more SMs 732. The TTU 738 includes hardware components configured toperform (or accelerate) operations commonly utilized in ray tracingalgorithms. A goal of the TTU 738 is to accelerate operations used inray tracing to such an extent that it brings the power of ray tracing toreal-time graphics application (e.g., games), enabling high-qualityshadows, reflections, and global illumination. Results produced by theTTU 738 may be used together with or as an alternative to other graphicsrelated operations performed in the GPU 730.

More specifically, SMs 732 and the TTU 738 may cooperate to cast raysinto a 3D model and determine whether and where that ray intersects themodel's geometry. Ray tracing directly simulates light traveling througha virtual environment or scene. The results of the ray intersectionstogether with surface texture, viewing direction, and/or lightingconditions are used to determine pixel color values. Ray tracingperformed by SMs 732 working with TTU 738 allows for computer-generatedimages to capture shadows, reflections, and refractions in ways that canbe indistinguishable from photographs or video of the real world. Sinceray tracing techniques are even more computationally intensive thanrasterization due in part to the large number of rays that need to betraced, the TTU 738 is capable of accelerating in hardware certain ofthe more computationally-intensive aspects of that process.

Given a BVH constructed as described above, the TTU 738 performs a treesearch where each node in the tree visited by the ray has a boundingvolume for each descendent branch or leaf, and the ray only visits thedescendent branches or leaves whose corresponding bound volume itintersects. In this way, TTU 738 explicitly tests only a small number ofprimitives for intersection, namely those that reside in leaf nodesintersected by the ray. In the example non-limiting embodiments, the TTU738 accelerates both tree traversal (including the ray-volume tests) andray-primitive tests. As part of traversal, it can also handle at leastone level of instance transforms, transforming a ray from world-spacecoordinates into the coordinate system of an instanced mesh. In theexample non-limiting embodiments, the TTU 738 does all of this in MIMDfashion, meaning that rays are handled independently once inside theTTU.

In the example non-limiting embodiments, the TTU 738 operates as aservant (coprocessor) to the SMs (streaming multiprocessors) 732. Inother words, the TTU 738 in example non-limiting embodiments does notoperate independently, but instead follows the commands of the SMs 732to perform certain computationally-intensive ray tracing related tasksmuch more efficiently than the SMs 732 could perform themselves. Inother embodiments or architectures, the TTU 138 could have more or lessautonomy.

In the examples shown, the TTU 738 receives commands via SM 732instructions and writes results back to an SM register file. For manyuse cases (e.g., opaque triangles with at most two level of instancing),the TTU 738 can service the ray tracing query without furtherinteraction with the SM 732. More complicated queries (e.g., involvingalpha-tested triangles, primitives other than triangles, or more thantwo levels of instancing) may require multiple round trips (although thetechnology herein reduces the need for such “round trips” for certainkinds of geometry by providing the TTU 738 with enhanced capabilities toautonomously perform ray-bounding-volume intersection testing withoutthe need to ask the calling SM for help). In addition to tracing rays,the TTU 738 is capable of performing more general spatial queries wherean AABB or the extruded volume between two AABBs (which we call a“beam”) takes the place of the ray. Thus, while the TTU 738 isespecially adapted to accelerate ray tracing related tasks, it can alsobe used to perform tasks other than ray tracing.

The TTU 738 thus autonomously performs a test of each ray against a widerange of bounding volumes, and can cull any bounding volumes that don'tintersect with that ray. Starting at a root node that bounds everythingin the scene, the traversal co-processor tests each ray against smaller(potentially overlapping) child bounding volumes which in turn bound thedescendent branches of the BVH. The ray follows the child pointers forthe bounding volumes the ray hits to other nodes until the leaves orterminal nodes (volumes) of the BVH are reached.

Once the TTU 738 traverses the acceleration data structure to reach aterminal or “leaf” node (which may be represented by one or multiplebounding volumes) that intersects the ray and contains a geometricprimitive, it performs an accelerated ray-primitive intersection test todetermine whether the ray intersects that primitive (and thus the objectsurface that primitive defines). The ray-primitive test can provideadditional information about primitives the ray intersects that can beused to determine the material properties of the surface required forshading and visualization. Recursive traversal through the accelerationdata structure enables the traversal co-processor to discover all objectprimitives the ray intersects, or the closest (from the perspective ofthe viewpoint) primitive the ray intersects (which in some cases is theonly primitive that is visible from the viewpoint along the ray). Seee.g., Lefrancois et al, NVIDIA Vulkan Ray Tracing Tutorial, December2019, https://developer.nvidia.com/rtx/raytracing/vkray

As mentioned above, the TTU 138 also accelerates the transform of eachray from world space into object space to obtain finer and finerbounding box encapsulations of the primitives and reduce the duplicationof those primitives across the scene. As described above, objectsreplicated many times in the scene at different positions, orientationsand scales can be represented in the scene as instance nodes whichassociate a bounding box and leaf node in the world space BVH with atransformation that can be applied to the world-space ray to transformit into an object coordinate space, and a pointer to an object-spaceBVH. This avoids replicating the object space BVH data multiple times inworld space, saving memory and associated memory accesses. The instancetransform increases efficiency by transforming the ray into object spaceinstead of requiring the geometry or the bounding volume hierarchy to betransformed into world (ray) space and is also compatible withadditional, conventional rasterization processes that graphicsprocessing performs to visualize the primitives.

Example Ray Tracing Processes

FIG. 8 shows an exemplary ray tracing shading pipeline 800 that may beperformed by SM 732 and accelerated by TTU 738. The ray tracing shadingpipeline 800 starts by an SM 732 invoking ray generation 810 and issuinga corresponding ray tracing request to the TTU 738. The ray tracingrequest identifies a single ray cast into the scene and asks the TTU 738to search for intersections with an acceleration data structure the SM732 also specifies. The TTU 738 traverses (FIG. 8 block 820) theacceleration data structure to determine intersections or potentialintersections between the ray and the volumetric subdivisions andassociated triangles the acceleration data structure represents.Potential intersections can be identified by finding bounding volumes inthe acceleration data structure that are intersected by the ray.Descendants of non-intersected bounding volumes need not be examined.

For triangles within intersected bounding volumes, the TTU 738ray-primitive test block 1020 performs an intersection 830 process todetermine whether the ray intersects the primitives. The TTU 738 returnsintersection information to the SM 732, which may perform an “any hit”shading operation 840 in response to the intersection determination. Forexample, the SM 732 may perform (or have other hardware perform) atexture lookup for an intersected primitive and decide based on theappropriate texel's value how to shade a pixel visualizing the ray. TheSM 732 keeps track of such results since the TTU 738 may return multipleintersections with different geometry in the scene in arbitrary order.

FIG. 9 is a flowchart summarizing example ray tracing operations the TTU738 performs as described above in cooperation with SM(s) 732. The FIG.9 operations are performed by TTU 738 in cooperation with itsinteraction with an SM 732. The TTU 738 may thus receive theidentification of a ray from the SM 732 and traversal state enumeratingone or more nodes in one or more BVH's that the ray must traverse. TheTTU 738 determines which bounding volumes of a BVH data structure theray intersects (the “ray-complet” test 912). The TTU 738 can alsosubsequently determine whether the ray intersects one or more primitivesin the intersected bounding volumes and which triangles are intersected(the “ray-primitive test” 920)— or the SM 732 can perform this test insoftware if it is too complicated for the TTU to perform itself. Inexample non-limiting embodiments, complets specify root or interiornodes (i.e., volumes) of the bounding volume hierarchy with childrenthat are other complets or leaf nodes of a single type per complet.

First, the TTU 738 inspects the traversal state of the ray. If a stackthe TTU 738 maintains for the ray is empty, then traversal is complete.If there is an entry on the top of the stack, the traversal co-processor738 issues a request to the memory subsystem to retrieve that node. Thetraversal co-processor 738 then performs a bounding box test 912 todetermine if a bounding volume of a BVH data structure is intersected bya particular ray the SM 732 specifies (step 912, 914). If the boundingbox test determines that the bounding volume is not intersected by theray (“No” in step 914), then there is no need to perform any furthertesting for visualization and the TTU 738 can return this result to therequesting SM 732. This is because if a ray misses a bounding volume (asin FIG. 1A with respect to bounding volume 110), then the ray will missall other smaller bounding volumes inside the bounding volume beingtested and any primitives that bounding volume contains.

If the bounding box test performed by the TTU 738 reveals that thebounding volume is intersected by the ray (“Yes” in Step 914), then theTTU determines if the bounding volume can be subdivided into smallerbounding volumes (step 918). In one example embodiment, the TTU 738isn't necessarily performing any subdivision itself. Rather, each nodein the BVH has one or more children (where each child is a leaf or abranch in the BVH). For each child, there is one or more boundingvolumes and a pointer that leads to a branch or a leaf node. When a rayprocesses a node using TTU 738, it is testing itself against thebounding volumes of the node's children. The ray only pushes stackentries onto its stack for those branches or leaves whose representativebounding volumes were hit. When a ray fetches a node in the exampleembodiment, it doesn't test against the bounding volume of the node—ittests against the bounding volumes of the node's children. The TTU 738pushes nodes whose bounding volumes are hit by a ray onto the ray'straversal stack (e.g. traversal stack 1302 in FIG. 13 ) in an orderdetermined by ray configuration. For example, it is possible to pushnodes onto the traversal stack in the order the nodes appear in memory,or in the order that they appear along the length of the ray, or in someother order. If there are further subdivisions of the bounding volume(“Yes” in step 918), then those further subdivisions of the boundingvolume are accessed and the bounding box test is performed for each ofthe resulting subdivided bounding volumes to determine which subdividedbounding volumes are intersected by the ray and which are not. In thisrecursive process, some of the bounding volumes may be eliminated bytest 914 while other bounding volumes may result in still further andfurther subdivisions being tested for intersection by TTU 738recursively applying steps 912-918.

Once the TTU 738 determines that the bounding volumes intersected by theray are leaf nodes (“No” in step 918), the TTU 738 and/or SM 732performs a primitive (e.g., triangle) intersection test 920 to determinewhether the ray intersects primitives in the intersected boundingvolumes and which primitives the ray intersects. The TTU 738 thusperforms a depth-first traversal of intersected descendent branch nodesuntil leaf nodes are reached. The TTU 738 processes the leaf nodes. Ifthe leaf nodes are primitive ranges, the TTU 738 or the SM 732 teststhem against the ray. If the leaf nodes are instance nodes, the TTU 738or the SM 732 applies the instance transform. If the leaf nodes are itemranges, the TTU 738 returns them to the requesting SM 732. In theexample non-limiting embodiments, the SM 732 can command the TTU 738 toperform different kinds of ray-primitive intersection tests and reportdifferent results depending on the operations coming from an application(or an software stack the application is running on) and relayed by theSM to the TTU. For example, the SM 732 can command the TTU 738 to reportthe nearest visible primitive revealed by the intersection test, or toreport all primitives the ray intersects irrespective of whether theyare the nearest visible primitive. The SM 732 can use these differentresults for different kinds of visualization. Or the SM 732 can performthe ray-primitive intersection test itself once the TTU 738 has reportedthe ray-complet test results. Once the TTU 738 is done processing theleaf nodes, there may be other branch nodes (pushed earlier onto theray's stack) to test.

Example Non-Limiting TTU 738 Hardware Implementation

FIG. 10 shows an example simplified block diagram of TTU 738 includinghardware configured to perform accelerated traversal operations asdescribed above. In some embodiments, the TTU 738 may perform adepth-first traversal of a bounding volume hierarchy using a short stacktraversal with intersection testing of supported leaf node primitivesand mid-traversal return of alpha primitives and unsupported leaf nodeprimitives (items). The TTU 738 includes dedicated hardware to determinewhether a ray intersects bounding volumes and dedicated hardware todetermine whether a ray intersects primitives of the tree datastructure.

In more detail, TTU 738 includes an intersection management block 1022,a ray management block 1030 and a stack management block 1040. Each ofthese blocks (and all of the other blocks in FIG. 10 ) may constitutededicated hardware implemented by logic gates, registers,hardware-embedded lookup tables or other combinatorial logic, etc.

The ray management block 1030 is responsible for managing informationabout and performing operations concerning a ray specified by an SM 732to the ray management block. The stack management block 1040 works inconjunction with traversal logic 1012 to manage information about andperform operations related to traversal of a BVH acceleration datastructure. Traversal logic 1012 is directed by results of a ray-complettest block 1010 that tests intersections between the ray indicated bythe ray management block 1030 and volumetric subdivisions represented bythe BVH, using instance transforms as needed. The ray-complet test block1010 retrieves additional information concerning the BVH from memory 740via an L0 complet cache 1052 that is part of the TTU 738. The results ofthe ray-complet test block 1010 informs the traversal logic 1012 as towhether further recursive traversals are needed. The stack managementblock 740 maintains stacks to keep track of state information as thetraversal logic 1012 traverses from one level of the BVH to another,with the stack management block 1040 pushing items onto the stack as thetraversal logic traverses deeper into the BVH and popping items from thestack as the traversal logic traverses upwards in the BVH. The stackmanagement block 1040 is able to provide state information (e.g.,intermediate or final results) to the requesting SM 732 at any time theSM requests.

The intersection management block 1022 manages information about andperforms operations concerning intersections between rays andprimitives, using instance transforms as needed. The ray-primitive testblock 1020 retrieves information concerning geometry from memory 140 onan as-needed basis via an L0 primitive cache 1054 that is part of TTU138. The intersection management block 1022 is informed by results ofintersection tests the ray-primitive test and transform block 1020performs. Thus, the ray-primitive test and transform block 1-20 providesintersection results to the intersection management block 1022, whichreports geometry hits and intersections to the requesting SM 732.

A Stack Management Unit 1040 inspects the traversal state to determinewhat type of data needs to be retrieved and which data path (complet orprimitive) will consume it. The intersections for the bounding volumesare determined in the ray-complet test path of the TTU 738 including oneor more ray-complet test blocks 1010 and one or more traversal logicblocks 1012. A complet specifies root or interior nodes of a boundingvolume. Thus, a complet may define one or more bounding volumes for theray-complet test. In example embodiments herein, a complet may define aplurality of “child” bounding volumes that (whether or not theyrepresent leaf nodes) that don't necessarily each have descendants butwhich the TTU will test in parallel for ray-bounding volume intersectionto determine whether geometric primitives associated with the pluralityof bounding volumes need to be tested for intersection.

The ray-complet test path of the TTU 738 identifies which boundingvolumes are intersected by the ray. Bounding volumes intersected by theray need to be further processed to determine if the primitivesassociated with the intersected bounding volumes are intersected. Theintersections for the primitives are determined in the ray-primitivetest path including one or more ray-primitive test and transform blocks1020 and one or more intersection management blocks 1022.

The TTU 738 receives queries from one or more SMs 732 to perform treetraversal operations. The query may request whether a ray intersectsbounding volumes and/or primitives in a BVH data structure. The querymay identify a ray (e.g., origin, direction, and length of the ray) anda BVH data structure and traversal state (short stack) which includesone or more entries referencing nodes in one or more Bounding VolumeHierarchies that the ray is to visit. The query may also includeinformation for how the ray is to handle specific types of intersectionsduring traversal. The ray information may be stored in the raymanagement block 1030. The stored ray information (e.g., ray length) maybe updated based on the results of the ray-primitive test.

The TTU 738 may request the BVH data structure identified in the queryto be retrieved from memory outside of the TTU 738. Retrieved portionsof the BVH data structure may be cached in the level-zero (LO) cache1050 within the TTU 738 so the information is available for othertime-coherent TTU operations, thereby reducing memory 1040 accesses.Portions of the BVH data structure needed for the ray-complet test maybe stored in a L0 complet cache 1052 and portions of the BVH datastructure needed for the ray-primitive test may be stored in an L0primitive cache 1054.

After the complet information needed for a requested traversal step isavailable in the complet cache 1052, the ray-complet test block 1010determines bounding volumes intersected by the ray. In performing thistest, the ray may be transformed from the coordinate space of thebounding volume hierarchy to a coordinate space defined relative to acomplet. The ray is tested against the bounding boxes associated withthe child nodes of the complet. In the example non-limiting embodiment,the ray is not tested against the complet's own bounding box because (1)the TTU 738 previously tested the ray against a similar bounding boxwhen it tested the parent bounding box child that referenced thiscomplet, and (2) a purpose of the complet bounding box is to define alocal coordinate system within which the child bounding boxes can beexpressed in compressed form. If the ray intersects any of the childbounding boxes, the results are pushed to the traversal logic todetermine the order that the corresponding child pointers will be pushedonto the traversal stack (further testing will likely require thetraversal logic 1012 to traverse down to the next level of the BVH).These steps are repeated recursively until intersected leaf nodes of theBVH are encountered

The ray-complet test block 1010 may provide ray-complet intersections tothe traversal logic 1012. Using the results of the ray-complet test, thetraversal logic 1012 creates stack entries to be pushed to the stackmanagement block 1040. The stack entries may indicate internal nodes(i.e., a node that includes one or more child nodes) that need to befurther tested for ray intersections by the ray-complet test block 1010and/or triangles identified in an intersected leaf node that need to betested for ray intersections by the ray-primitive test and transformblock 1020. The ray-complet test block 1010 may repeat the traversal oninternal nodes identified in the stack to determine all leaf nodes inthe BVH that the ray intersects. The precise tests the ray-complet testblock 1010 performs will in the example non-limiting embodiment bedetermined by mode bits, ray operations (see below) and culling of hits,and the TTU 738 may return intermediate as well as final results to theSM 732.

Ray-Primitive Intersection Testing

Referring again to FIG. 11 , the TTU 738 also has the ability toaccelerate intersection tests that determine whether a ray intersectsparticular geometry or primitives. For some cases, the geometry issufficiently complex (e.g., defined by curves or other abstractconstructs as opposed to e.g., vertices) that TTU 738 in someembodiments may not be able to help with the ray-primitive intersectiontesting. In such cases, the TTU 738 simply reports the ray-completintersection test results to the SM 732, and the SM 732 performs theray-primitive intersection test itself. In other cases (e.g.,triangles), the TTU 732 can perform the ray-triangle intersection testitself, thereby further increasing performance of the overall raytracing process. For sake of completeness, the following describes howthe TTU 738 can perform or accelerate the ray-primitive intersectiontesting.

As explained above, leaf nodes found to be intersected by the rayidentify (enclose) primitives that may or may not be intersected by theray. One option is for the TTU 738 to provide e.g., a range of geometryidentified in the intersected leaf nodes to the SM 732 for furtherprocessing. For example, the SM 732 may itself determine whether theidentified primitives are intersected by the ray based on theinformation the TTU 738 provides as a result of the TTU traversing theBVH. To offload this processing from the SM 732 and thereby accelerateit using the hardware of the TTU 738, the stack management block 1040may issue requests for the ray-primitive and transform block 1020 toperform a ray-primitive test for the primitives within intersected leafnodes the TTU's ray-complet test block 1010 identified. In someembodiments, the SM 732 may issue a request for the ray-primitive testto test a specific range of primitives and transform block 1020irrespective of how that geometry range was identified.

After making sure the primitive data needed for a requestedray-primitive test is available in the primitive cache 1054, theray-primitive and transform block 1020 may determine primitives that areintersected by the ray using the ray information stored in the raymanagement block 1030. The ray-primitive test block 1020 provides theidentification of primitives determined to be intersected by the ray tothe intersection management block 1022.

The intersection management block 1022 can return the results of theray-primitive test to the SM 732. The results of the ray-primitive testmay include identifiers of intersected primitives, the distance ofintersections from the ray origin and other information concerningproperties of the intersected primitives. In some embodiments, theintersection management block 1022 may modify an existing ray-primitivetest (e.g., by modifying the length of the ray) based on previousintersection results from the ray-primitive and transform block 1020.

The intersection management block 1022 may also keep track of differenttypes of primitives. For example, the different types of trianglesinclude opaque triangles that will block a ray when intersected andalpha triangles that may or may not block the ray when intersected ormay require additional handling by the SM. Whether a ray is blocked ornot by a transparent triangle may for example depend on texture(s)mapped onto the triangle, area of the triangle occupied by the textureand the way the texture modifies the triangle. For example, transparency(e.g., stained glass) in some embodiments requires the SM 732 to keeptrack of transparent object hits so they can be sorted and shaded inray-parametric order, and typically don't actually block the ray.Meanwhile, alpha “trimming” allows the shape of the primitive to betrimmed based on the shape of a texture mapped onto the primitive—forexample, cutting a leaf shape out of a triangle. (Note that in rastergraphics, transparency is often called “alpha blending” and trimming iscalled “alpha test”). In other embodiments, the TTU 738 can pushtransparent hits to queues in memory for later handling by the SM 732and directly handle trimmed triangles by sending requests to the textureunit. Each triangle may include a designator to indicate the triangletype. The intersection management block 1022 is configured to maintain aresult queue for tracking the different types of intersected triangles.For example, the result queue (e.g. result queue 1410 in FIG. 14 ) maystore one or more intersected opaque triangle identifiers in one queue1412 and one or more transparent triangle identifiers in another queue1414.

For opaque triangles, the ray intersection for less complex geometry canbe fully determined in the TTU 738 because the area of the opaquetriangle blocks the ray from going past the surface of the triangle. Fortransparent triangles, ray intersections cannot in some embodiments befully determined in the TTU 738 because TTU 738 performs theintersection test based on the geometry of the triangle and may not haveaccess to the texture of the triangle and/or area of the triangleoccupied by the texture (in other embodiments, the TTU may be providedwith texture information by the texture mapping block of the graphicspipeline). To fully determine whether the triangle is intersected,information about transparent triangles the ray-primitive and transformblock 1020 determines are intersected may be sent to the SM 732, for theSM to make the full determination as to whether the triangle affectsvisibility along the ray.

The SM 732 can resolve whether or not the ray intersects a textureassociated with the transparent triangle and/or whether the ray will beblocked by the texture. The SM 732 may in some cases send a modifiedquery to the TTU 738 (e.g., shortening the ray if the ray is blocked bythe texture) based on this determination. In one embodiment, the TTU 738may be configured to return all triangles determined to intersect theray to the SM 732 for further processing. Because returning everytriangle intersection to the SM 732 for further processing is costly interms of interface and thread synchronization, the TTU 738 may beconfigured to hide triangles which are intersected but are provablycapable of being hidden without a functional impact on the resultingscene. For example, because the TTU 738 is provided with triangle typeinformation (e.g., whether a triangle is opaque or transparent), the TTU738 may use the triangle type information to determine intersectedtriangles that are occluded along the ray by another intersecting opaquetriangle and which thus need not be included in the results because theywill not affect the visibility along the ray. If the TTU 738 knows thata triangle is occluded along the ray by an opaque triangle, the occludedtriangle can be hidden from the results without impact on visualizationof the resulting scene.

The intersection management block 1022 may include a result queue forstoring hits that associate a triangle ID and information about thepoint where the ray hit the triangle. When a ray is determined tointersect an opaque triangle, the identity of the triangle and thedistance of the intersection from the ray origin can be stored in theresult queue. If the ray is determined to intersect another opaquetriangle, the other intersected opaque triangle can be omitted from theresult if the distance of the intersection from the ray origin isgreater than the distance of the intersected opaque triangle alreadystored in the result queue. If the distance of the intersection from theray origin is less than the distance of the intersected opaque trianglealready stored in the result queue, the other intersected opaquetriangle can replace the opaque triangle stored in the result queue.After all of the triangles of a query have been tested, the opaquetriangle information stored in the result queue and the intersectioninformation may be sent to the SM 732.

In some embodiments, once an opaque triangle intersection is identified,the intersection management block 1022 may shorten the ray stored in theray management block 1030 so that bounding volumes (which may includetriangles) behind the intersected opaque triangle (along the ray) willnot be identified as intersecting the ray.

The intersection management block 1022 may store information aboutintersected transparent triangles in a separate queue. The storedinformation about intersected transparent triangles may be sent to theSM 732 for the SM to resolve whether or not the ray intersects a textureassociated with the triangle and/or whether the texture blocks the ray.The SM may return the results of this determination to the TTU 738and/or modify the query (e.g., shorten the ray if the ray is blocked bythe texture) based on this determination.

As discussed above, the TTU 138 allows for quick traversal of anacceleration data structure (e.g., a BVH) to determine which primitives(e.g., triangles used for generating a scene) in the data structure areintersected by a query data structure (e.g., a ray). For example, theTTU 738 may determine which triangles in the acceleration data structureare intersected by the ray and return the results to the SM 732.However, returning to the SM 732 a result on every triangle intersectionis costly in terms of interface and thread synchronization. The TTU 738provides a hardware logic configured to hide those items or triangleswhich are provably capable of being hidden without a functional impacton the resulting scene. The reduction in returns of results to the SMand synchronization steps between threads greatly improves the overallperformance of traversal. The example non-limiting embodiments of theTTU 738 disclosed in this application provides for some of theintersections to be discarded within the TTU 738 without SM 732intervention so that less intersections are returned to the SM 732 andthe SM 132 does not have to inspect all intersected triangles or itemranges.

Example Instancing Pipeline Implementation by TTU 738 and SM 732

The following describes how TTU 738 in example embodiments performsinstancing and associated transforms.

The FIG. 12A more detailed diagram of a ray-tracing pipeline flowchartshows the data flow and interaction between components for arepresentative use case: tracing rays against a scene containinggeometric primitives, with instance transformations handled in hardware.In one example non-limiting embodiment, the ray-tracing pipeline of FIG.12A is essentially software-defined (which in example embodiments meansit is determined by the SMs 732) but makes extensive use of hardwareacceleration by TTU 738. Key components include the SM 732 (and the restof the compute pipeline), the TTU 738 (which serves as a coprocessor toSM), and the L1 cache and downstream memory system, from which the TTUfetches BVH and triangle data.

The pipeline shown in FIG. 12A shows that bounding volume hierarchycreation 1202 can be performed ahead of time by a development system. Italso shows that ray creation and distribution 1204 are performed orcontrolled by the SM 732 or other software in the example embodiment, asshading (which can include lighting and texturing). The example pipelineincludes a “top level” BVH tree traversal 1206, ray transformation 1214,“bottom level” BVH tree traversal 1218, and a ray/triangle (or otherprimitive) intersection 1226 that are each performed by the TTU 738.These do not have to be performed in the order shown, as handshakingbetween the TTU 738 and the SM 732 determines what the TTU 738 does andin what order.

The SM 732 presents one or more rays to the TTU 738 at a time. Each raythe SM 732 presents to the TTU 738 for traversal may include the ray'sgeometric parameters, traversal state, and the ray's ray flags, modeflags and ray operations information. In an example embodiment, a rayoperation (RayOp) provides or comprises an auxiliary arithmetic and/orlogical test to suppress, override, and/or allow storage of anintersection. The traversal stack may also be used by the SM 732 tocommunicate certain state information to the TTU 738 for use in thetraversal. A new ray query may be started with an explicit traversalstack. For some queries, however, a small number of stack initializersmay be provided for beginning the new query of a given type, such as,for example: traversal starting from a complet; intersection of a raywith a range of triangles; intersection of a ray with a range oftriangles, followed by traversal starting from a complet; vertex fetchfrom a triangle buffer for a given triangle, etc. In some embodiments,using stack initializers instead of explicit stack initializationimproves performance because stack initializers require fewer streamingprocessor registers and reduce the number of parameters that need to betransmitted from the streaming processor to the TTU.

In the example embodiment, a set of mode flags the SM 732 presents witheach query (e.g., ray) may at least partly control how the TTU 738 willprocess the query when the query intersects the bounding volume of aspecific type or intersects a primitive of a specific primitive type.The mode flags the SM 732 provides to the TTU 738 enable the ability bythe SM and/or the application to e.g., through a RayOp, specify anauxiliary arithmetic or logical test to suppress, override, or allowstorage of an intersection. The mode flags may for example enabletraversal behavior to be changed in accordance with such aspects as, forexample, a depth (or distance) associated with each bounding volumeand/or primitive, size of a bounding volume or primitive in relation toa distance from the origin or the ray, particular instances of anobject, etc. This capability can be used by applications to dynamicallyand/or selectively enable/disable sets of objects for intersectiontesting versus specific sets or groups of queries, for example, to allowfor different versions of models to be used when application statechanges (for example, when doors open or close) or to provide differentversions of a model which are selected as a function of the length ofthe ray to realize a form of geometric level of detail, or to allowspecific sets of objects from certain classes of rays to make somelayers visible or invisible in specific views.

In addition to the set of mode flags which may be specified separatelyfor the ray-complet intersection and for ray-primitive intersections,the ray data structure may specify other RayOp test related parameters,such as ray flags, ray parameters and a RayOp test. The ray flags can beused by the TTU 738 to control various aspects of traversal behavior,back-face culling, and handling of the various child node types, subjectto a pass/fail status of an optional RayOp test. RayOp tests addflexibility to the capabilities of the TTU 738, at the expense of somecomplexity. The TTU 138 reserves a “ray slot” for each active ray it isprocessing, and may store the ray flags, mode flags and/or the RayOpinformation in the corresponding ray slot buffer within the TTU duringtraversal.

In the example shown in FIG. 12A, the TTU 738 performs a top level treetraversal 1206 and a bottom level tree traversal 1218. In the exampleembodiment, the two level traversal of the BVH enables fast ray tracingresponses to dynamic scene changes.

In some embodiments, upon entry to top level tree traversal, or in thetop level tree traversal, an optional instance node 1205 specifying atop level transform is encountered in the BVH. The instance node 1205,if it exists, indicates to the TTU that the subtree rooted at theinstance node 1205 is aligned to an alternate world space coordinatesystem for which the transform from the world space is defined in theinstance node 1205. Top level instance nodes and their use are describedin U.S. application Ser. No. 16/897,745, titled “Ray Tracing HardwareAcceleration with Alternative World Space Transforms” which is hereinincorporated by reference in its entirety.

The top level of the acceleration structure (TLAS) contains geometry inworld space coordinates and the bottom level of the accelerationstructure (BLAS) contains geometry in object space coordinates. The TTUmaintains ray state and stack state separately for the TLAS traversaland the BLAS traversal because they are effectively independenttraversals.

As described above the SM informs the TTU the location in the BVH forstarting a ray traversal upon launching a new ray query or relaunching aray query by including a stack initialization complet in the ray querytransmitted to the TTU. The stack initialization complet includes apointer to the root of the subtree that is to be traversed.

Ray transformation 1214 provides the appropriate transition from the toplevel tree traversal 1206 to the bottom level tree traversal 1218 bytransforming the ray, which may be used in the top level traversal in afirst coordinate space (e.g., world space), to a different coordinatespace (e.g., object space) of the BVH of the bottom level traversal. Anexample BVH traversal technique using a two level traversal is describedin previous literature, see, e.g., Woop, “A Ray Tracing HardwareArchitecture for Dynamic Scenes”, Universitat des Saarlandes, 2004, butembodiments are not limited thereto.

Example Top Level Tree Traversal

The top level tree traversal 1206 by TTU 738 receives complets from theL1 cache 1212, and provides an instance to the ray transformation 1214for transformation, or a miss/end output 1213 to the SM 732 for closesthit shader 1215 processing by the SM (this block can also operaterecursively based on non-leaf nodes/no hit conditions). In the top leveltree traversal 1206, a next complet fetch step 1208 fetches the nextcomplet to be tested for ray intersection in step 1210 from the memoryand/or cache hierarchy and ray-bounding volume intersection testing isdone on the bounding volumes in the fetched complet.

As described above, an instance node connects one BVH to another BVHwhich is in a different coordinate system. When a child of theintersected bounding volume is an instance node, the ray transformation1214 is able to retrieve an appropriate transform matrix from the L1cache 1216. The TTU 738, using the appropriate transform matrix,transforms the ray to the coordinate system of the child BVH. U.S.patent application Ser. No. 14/697,480, which is already incorporated byreference, describes transformation nodes that connect a first set ofnodes in a tree to a second set of nodes where the first and second setsof nodes are in different coordinate systems. The instance nodes inexample embodiments may be similar to the transformation nodes in U.S.application Ser. No. 14/697,480. In an alternative, non-instancing modeof TTU 738 shown in FIG. 12B, the TTU does not execute a “bottom” leveltree traversal 1018 and noninstanced tree BVH traversals are performedby blocks 1208, 1210 e.g., using only one stack. The TTU 738 can switchbetween the FIG. 12A instanced operations and the FIG. 12B non-instancedoperations based on what it reads from the BVH and/or query type. Forexample, a specific query type may restrict the TTU to use just thenon-instanced operations. In such a query, any intersected instancenodes would be returned to the SM.

In some non-limiting embodiments, ray-bounding volume intersectiontesting in step 1210 is performed on each bounding volume in the fetchedcomplet before the next complet is fetched. Other embodiments may useother techniques, such as, for example, traversing the top leveltraversal BVH in a depth-first manner. U.S. Pat. No. 9,582,607, alreadyincorporated by reference, describes one or more complet structures andcontents that may be used in example embodiments. U.S. Pat. No.9,582,607 also describes an example traversal of complets.

When a bounding volume is determined to be intersected by the ray, thechild bounding volumes (or references to them) of the intersectedbounding volume are kept track of for subsequent testing forintersection with the ray and for traversal. In example embodiments, oneor more stack data structures is used for keeping track of childbounding volumes to be subsequently tested for intersection with theray. In some example embodiments, a traversal stack of a small size maybe used to keep track of complets to be traversed by operation of thetop level tree traversal 1206, and primitives to be tested forintersection, and a larger local stack data structure can be used tokeep track of the traversal state in the bottom level tree traversal1218. FIG. 13 shows an example traversal stack 1302 with bottom stackentry 1304 and top stack entry 1306.

Example Bottom Level Tree Traversal

In the bottom level tree traversal 1218, a next complet fetch step 1222fetches the next complet to be tested for ray intersection in step 1224from the memory and/or cache hierarchy 1220 and ray-bounding volumeintersection testing is done on the bounding volumes in the fetchedcomplet. The bottom level tree traversal, as noted above, may includecomplets with bounding volumes in a different coordinate system than thebounding volumes traversed in the upper level tree traversal. The bottomlevel tree traversal also receives complets from the L1 cache and canoperate recursively or iteratively within itself based onnon-leaf/no-hit conditions and also with the top level tree traversal1206 based on miss/end detection. Intersections of the ray with thebounding volumes in the lower level BVH may be determined with the raytransformed to the coordinate system of the lower level completretrieved. The leaf bounding volumes found to be intersected by the rayin the lower level tree traversal are then provided to the ray/triangleintersection 1226.

The leaf outputs of the bottom level tree traversal 1218 are provided tothe ray/triangle intersection 1226 (which has L0 cache access as well asability to retrieve triangles via the L1 cache 1228). The L0 complet andtriangle caches may be small read-only caches internal to the TTU 138.The ray/triangle intersection 1226 may also receive leaf outputs fromthe top level tree traversal 1206 when certain leaf nodes are reachedwithout traversing an instanced BVH.

After all the primitives in the primitive range have been processed, theIntersection Management Unit inspects the state of the result Queue(e.g. result queue 1410 in FIG. 14 ) and crafts packets to send to theStack Management Unit and/or Ray Management Unit to update the ray'sattributes and traversal state, set up the ray's next traversal step,and/or return the ray to the SM 732 (if necessary). If the result queuecontains opaque 1412 or alpha 1414 intersections found during theprocessing of the primitive range then the Intersection Management Unitsignals the parametric length (t) of the nearest opaque intersection inthe result queue to the ray management unit to record as the ray's tmaxto shorten the ray. To update the traversal state to set up the ray'snext traversal step the Intersection Management Unit signals to theStack Management Unit whether an opaque intersection from the primitiverange is present in the resultQueue, whether one or more alphaintersections are present in the result queue, whether the resultQueueis full, whether additional alpha intersections were found in theprimitive range that have not been returned to the SM and which are notpresent in the resultQueue, and the index of the next alpha primitive inthe primitive range for the ray to test after the SM consumes thecontents of the resultQueue (the index of the next primitive in therange after the alpha primitive with the highest memory-order from thecurrent primitive range in the result queue).

When the Stack Management Unit 1040 receives the packet fromIntersection Management Unit 1022, the Stack Management Unit 1040inspects the packet to determine the next action required to completethe traversal step and start the next one. If the packet fromIntersection Management Unit 1022 indicates an opaque intersection hasbeen found in the primitive range and the ray mode bits indicate the rayis to finish traversal once any intersection has been found the StackManagement Unit 1040 returns the ray and its results queue to the SMwith traversal state indicating that traversal is complete (a done flagset and/or an empty top level and bottom level stack). If the packetfrom Intersection Management Unit 1022 indicates that there opaque oralpha intersection in the result queue and that there are remainingalpha intersections in the primitive range not present in the resultqueue that were encountered by the ray during the processing of theprimitive range that have not already been returned to the SM, the StackManagement Unit 1040 returns the ray and the result queue to the SM withtraversal state modified to set the cull opaque bit to prevent furtherprocessing of opaque primitives in the primitive range and the primitiverange starting index advanced to the first alpha primitive after thehighest alpha primitive intersection from the primitive range returnedto the SM in the ray's result queue. If the packet from IntersectionManagement Unit 1022 indicates that no opaque or alpha intersectionswere found when the ray processed the primitive range the StackManagement Unit 1040 pops the top of stack entry (corresponding to thefinished primitive range) off the active traversal stack. If the packetfrom Stack Management Unit 1040 indicates or that either there areopaque intersections in the result queue and the ray mode bits do notindicate that the ray is to finish traversal once any intersection hasbeen found and/or there are alpha intersections in the result queue, butthere were no remaining alpha intersections found in the primitive rangenot present in the result queue that have not already been returned tothe SM the Stack Management Unit 1040 pops the top of stack entry(corresponding to the finished primitive range) off the active traversalstack and modifies the contents of the result queue to indicate that allintersections present in the result queue come from a primitive rangewhose processing was completed.

If the active stack is the bottom stack, and the bottom stack is emptythe Stack Management Unit 1040 sets the active stack to the top stack.If the top stack is the active stack, and the active stack is empty,then the Stack Management Unit 1040 returns the ray and its result queueto the SM with traversal state indicating that traversal is complete (adone flag set and/or an empty top level and bottom level stack). If theactive stack contains one or more stack entries, then the StackManagement Unit 1040 inspects the top stack entry and starts the nexttraversal step. Testing of primitive and/or primitive ranges forintersections with a ray and returning results to the SM 732 aredescribed in co-pending U.S. application Ser. No. 16/101,148 entitled“Conservative Watertight Ray Triangle Intersection” and U.S. applicationSer. No. 16/101,196 entitled “Method for Handling Out-of-Order Opaqueand Alpha Ray/Primitive Intersections”, which are hereby incorporated byreference in their entireties.

Example Data Structures for Combined Ray Operation and Node Masking

During traversal of a BVH by a ray in the TTU, the traversal state forthe ray is maintained in the TTU. The traversal state may include astack of one or more entries which reference bounding volumes and/orcomplets in the tree structure which are to be fetched and testedagainst the ray. A traversal stack 1302 according to some embodiments isshown in FIG. 13 . The traversal stack 1302 may include any number ofstack entries. In some embodiments, the stack 1302 is limited to a smallnumber of entries (e.g., a “short stack” of 4 entries) so that theexchange of the stack between the TTU and SM can be made more efficient.In FIG. 13 , a bottom stack entry 1304 and a top stack entry 1306 areshown with one or more entries in between.

FIG. 14 shows an example results queue according to some embodiments. Aresult queue, as described elsewhere, is used for the TTU to transmitinformation about the intersections detected so far to the SM. In someembodiments, the result queue 1410 is small and may only accommodate anopaque primitive intersection result 1412 and/or one or more alphaprimitive intersection result 1414. However, in other embodiments, theresult queue may accommodate more entries representing detectedintersections.

FIG. 15A shows some example contents of a data structure correspondingto ray 1502, including a RayOp 1520. In some example embodiments, theray is generated in the SM 732 and the ray information is communicatedto the TTU 738 by way of registers in the SM. In example embodiments inwhich ray data is passed to the TTU via memory, data structure 1502, orpart thereof, may reside in a memory to be read by the TTU. Ray datastructure 1502 may include a ray identifier 1504 which may be assignedby the TTU or the SM to uniquely identify rays that are concurrentlybeing processed in the TTU, ray origin 1506, ray direction 1508, raystart (tmin) 1510 and end (tmax) 1512 parameters. According to someembodiments, the ray information 1502 may also include ray flags 1514,RCT mode flags 1516 (also referred to as RCT mode bits), RPT mode flags1518 (also referred to as RPT mode bits) and one or more ray operation(RayOps) specifications. Each RayOps specification may include a rayoperation opcode 1520 and ray test parameters (e.g., ray parameters A &B). These ray data attributes are described below.

As described below, a “RayOp” test is performed for each primitive orchild bounding box intersected by a ray using the ray's RayOp opcode,mode bits, and parameters A and B as well as one or more parameters(e.g., ChildType, “rval” parameter or “alpha” flag) specified with eachintersected complet child or primitive. In example embodiments, theChildType and rval parameters used in RayOp tests described below arespecified for each child in a complet, or for the complet as a whole,and the RayOp opcode, mode bits, and parameters A and B are specifiedfor each ray.

FIG. 15B shows example 4-bit opcodes used for the RayOp, according tosome embodiments. As shown, in the figure, some opcodes correspond toconstant operations such as for always perform the ray operation(ALWAYS), and for never performing the ray operation (NEVER). Some otheropcodes may correspond to integer operations such as, e.g., EQUAL, NOTEQUAL, GREATER etc., and yet other opcodes may correspond to operationsbased on the ray's length and like parameters (TMIN_LESS, TMAX_GEQUAL,etc.).

The rightmost column in FIG. 15B shows the logical or integer testperformed for the ray operation. The input parameters for the test mayinclude three constants according to some embodiments: two 8-bit or16-bit constants A and B provided by the ray, and an 8-bit “rval”provided by the node or child node in the acceleration data structure.Additionally, the tmin or the tmax of the ray and the child boundingvolume may be used in some tests.

In some embodiments, the rayop and associated data can be specifiedseparately for traversal in the TLAS and BLAS. According to exampleembodiments, the rayop that is set for the ray is the same for all nodesintersected by that ray, except when the FRA flag is set for a node inwhich case the rayop for that single node and ray is set to “ALWAYS”(always true, no action).

FIG. 15C shows example mode specifiers according to some embodiments.These include ray-complet test (RCT) mode specifiers and ray-triangletest (RTT) mode specifiers. The example mode specifiers may specifyactions to be performed based on the result (i.e. pass or fail) of therayop-based test and based on the node type in which the test isperformed. Example node types include an instance node (instance nodemode), an item range (item range mode), a triangle range (triangle rangemode), child complet (child complet mode), alpha triangle hit (alphatriangle hit mode), and an opaque triangle hit (opaque triangle hitmode). For each combination of a node type and a test result, one ormore specified actions are defined based on the value of the flag. Thespecified actions, indicated by the value of the respective modespecifiers, may include, for example, an action of continuing to processin the hardware coprocessor, an action of culling, an action ofreturning to the processing as a node reference, an action of returningto the processor, an action of treating an intersected triangle as analpha or opaque triangle, and the like. As an example, an instance nodewhich is intersected by a ray but that fails the ray op test would usethe “in_f” (modelInstanceNodeFail) bits that would indicate whether thatspecific instance node should be processed in the TTU 738, culled,returned to SM 732 as a node reference, or returned to SM 732 as is.

An example of a node data structure 1622 may hold RayOp-relatedinformation for a complet or bounding volume according to someembodiments is shown in FIG. 16A. According to some embodiments, datastructure 1622 may be stored in a memory by software, and the TTU mayeither access the data structure in the memory and/or may receive thedata structure into the TTU internal memory. The data structure 1622 mayinclude header information 1624, one or more override flags 1626 and anrval 1628. Header information may include geometric information, nodetype information etc., related to the node. Override flags 1626 and rvalparameter 1628 are described below. The rval flags 1628 and the overrideflags and parameters 1626 may be used by the RayOp test.

FIG. 16B illustrates an example data structure for a complet 1630. Theexample complet data structure includes a header area, and an areaholding the data for each child of the root node of that complet. Theheader portion includes x, y, z parameters defining the correspondingbounding volume. The header portion also includes the FRA flag 1633. Thedata in the complet for each child 1634 includes, as shown in FIG. 16C,an “rval” 1628 and an invert bit 1636.

Example Ray Operation with Force Ray Always (FRA) Flag not Set

FIG. 17 shows a flowchart of a programmable ray operation process 1700that may be performed when a ray—bounding volume intersection isdetected during ray tracing pipeline processing and it is determinedthat the FRA flag for that bounding volume is not set (e.g., value ofFRA flag is 0). For example, process 1700 may be performed when aray-bounding volume intersection is detected in step 1210 and/or 1224shown in FIG. 12A (e.g., in the top level traversal and/or in the bottomlevel traversal) with respect to process shown in FIG. 12A. Ray-boundingvolume intersection tests 1210 and/or 1224 may be performed in TTU 738in the ray-complet test (RCT) block 1110.

At step 1702, the ray-intersection test in a bounding box correspondingto a node determines that the bounding box/node is intersected by theray, and also that the corresponding FRA flag is not set (e.g. flagvalue is equal to 0), thus enabling the performing of the programmableray operation at the node.

The intersection detection at step 1702 may occur when testing aretrieved complet, or more specifically, testing a child bounding volumeincluded in the retrieved complet. According to example embodiments,when a complet is processed, the TTU may optionally perform the RayOptest on each child. In some embodiments, the RayOp test is run only onthe children whose corresponding bounding volume was intersected by theray.

Thus at step 1704, it is determined that the fetched complet has atleast one child, and at step 1706 the child bounding volumes areaccessed and tested. The child bounding volumes may be tested inparallel. In some embodiments, each retrieved complet has zero or oneparent complet and zero or more complet children and zero or more leafnode children. In some embodiments, each fetched complet references itsparent complet with a parent pointer or offset, encodes child pointersin compressed form, and provides a per-child structure containing achild bounding box and per-child data used by the RayOp test (e.g. Rval,invert RayOp result flag), and (in the case of leaf nodes) data used toaddress and process blocks of leaf nodes (e.g. item count, startingprimitive index, number of blocks in leaf, a flag indicating thepresence of alpha primitives). In some embodiments, processing steps1708-1714 may be performed in parallel for all children boundingvolumes. In some other embodiments, processing steps 1708-1714 may beperformed child-by-child, in parallel for groups of child boundingvolumes. etc.

Each of the child bounding volumes of the intersected parent arepotential traversal targets. In example embodiments, an instance node isa leaf node that points to the root node of another BVH. The RayOp testmay be performed on the child nodes of an intersected parent based uponthe child bounding volume information available in the already retrievedcomplet, before determining whether or not to retrieve the completscorresponding to the respective child nodes for traversal.

At step 1708, the RayOp test specified for the ray is performed withrespect to the accessed child bounding volume. As noted above inrelation to FIG. 15A-C, the RayOp opcode may be specified as part of theray data provided to the TTU 738 from the SM 732. In exampleembodiments, when the ray-bounding volume intersection is detected atray-complet test block 1010, the RCT block 1010 and/or the traversallogic block 1012 may perform the RayOp test based on parameters from theray, a child node of the intersected bounding volume, and optionally at-value associated with the intersection of the ray and the child nodeof the intersected bounding volume. The result of the test is used toselect between a pair of mode specifiers that determine how anintersected node of a given type should be handled during traversal.More specifically, the RayOp test specified by the particular RayOpopcode specified for the ray is performed using the ray's RayOp, A, Bparameters (mode specifiers) and the rval parameter specified for thechild bounding volume. In some embodiments, the RayOp test is performedonly for child bounding volumes that are themselves found to intersectthe ray. For example, when the RCT unit tests a ray against a complet,each of the complet's child bounding volumes are also tested forintersection with the ray and, for each child that is found to intersectthe ray, the RayOp test is performed. RayOp testing is described in U.S.patent application Ser. No. 16/101,180 titled “Query-Specific BehavioralModification of Tree Traversal”, which is already incorporated byreference, also assigned to Nvidia Corporation.

Child nodes can be either instance nodes, item ranges, triangle ranges,or complets. For any intersected child node in RCT traversal, the modesdictate whether it should be processed in the TTU (if possible), ignored(also referred to as, culled), or returned to the SM. If returned to theSM, there is the additional option to return either the node itself or areference to the node which means returning the parent node with anindex of the child node. The TTU may not further process item ranges,and the item ranges may be returned to the SM for further processing.

If a child node is not intersected by the ray in the RCT, then theresulting operation is a cull and neither the ray op nor mode flagsmatter.

Additionally for ray ops on triangle leaves, the ray op result is passedthrough to the triangle intersection test in the ray-triangle transformand test (RTT) unit, where the mode specifiers can be used to alterbehavior.

An example RayOp test may provide for testing a left hand side numericalvalue based on a ray parameter with respect to a particular arithmeticor logic operation, against a right hand side value based on a rayparameter and a parameter of the intersected node. The RayOp test may bean arithmetic or a logical computation that results in a true/falseoutput. The particular computation (e.g., the particular relationshipbetween the RayOp A and B parameters, the RayOp opcode and the rvalparameter) may be configurable, and/or may be preprogrammed in hardware.In some embodiments, each ray may specify one of a plurality of opcodescorresponding to respective RayOp tests. Thus, the RayOp test provides ahighly flexible technique by which rays can change the default raytracing behavior of the TTU 738 on an individual or group basis.

The RayOp tests may include any of, but are not limited to, thearithmetic and/or logic operations ALWAYS, NEVER, EQUAL, NOTEQUAL, LESS,LEQUAL, GREATER, GEQUAL, TMIN_LESS, TMIN_GEQUAL, TMAX_LESS, TMAX_GEQUAL,as opcodes. The opcode specified in a ray may, in some embodiments, beany logical or arithmetic operation.

For example, if the ray's RayOp opcode is defined in the ray informationprovided to the TTU as “EQUAL”, and the RayOp A and B parameters are 0x0and 0xFF, respectively, and the accessed child bounding volume's RayOprval is 0x1, the RayOp test may be “A EQUAL rval && B”. Thus, with theabove noted values for the various parameters and opcode, the RayOp testyields “0x00==0x1 && 0xFF”. Thus, (since this is false) the RayOp testin this example must return false. That is, in this particular example,the RayOp test fails for the ray and the accessed child bounding volume.

In some embodiments, the child bounding volume may also have an invert(“e.g., inv”) parameter associated with the RayOp testing. If the rayand/or child node also has an invert parameter associated with theRayOp, and the invert parameter is set to TRUE (e.g., 1), then thereturned RayOp result may be the inverse of the actual RayOp testresult. For example, if the invert parameter was set to TRUE, then theRayOp test in the above example would return TRUE. RayOps may becomparable to the Stencil Test in raster graphics, except that StencilTest has the ability to allow a fragment write to occur even when thefragment failed the Depth Test. In example embodiments, the RayOps donot have the capability to convert a missed complet child into a hitcomplet child, but in other embodiments the TTU could allowprogrammability so a RayOp could treat a miss as if it were a hit.

It is not necessary that the RayOp test has the parameters and theopcode arranged in a relationship such as “A EQUAL rval && B”. Exampleembodiments may have the parameters and the opcode arranged in anylogical or arithmetic relationship. In some embodiments, for example,the relationship may be of a form such as “TMIN_LESS rval” or “TMIN_LESSA & rval”, expressing a relationship between a specified area ofinterest and either the node parameter alone or a combination of the rayparameters and the node parameter. The example opcodes TMIN_LESS,TMIN_GEQUAL, TMAX_LESS, TMAX_GEQUAL all enable the RayOp test to bebased upon the intersection's start or end (e.g., TMIN and TMAX in theabove opcodes may represent the t values at the ray's entry to and exitfrom the intersected volume (e.g., bbox.tmin, bbox.tmax below),respectively), and to include aspects of either the tested node alone orthe tested node and the ray parameters A and/or B. For example, whenrval is encoded with a distance value for the node, “TMIN_LESS rval” mayrepresent a test such as “is the tested node at a distance less than thebeginning of the area of interest?”. Opcodes based on aspects of the rayother than start/end of the ray are also possible, and may be used forthe RayOp in other embodiments. In contrast to opcodes that encode anaspect of the ray's geometric properties, example opcodes ALWAYS, NEVER,EQUAL, NOTEQUAL, LESS, LEQUAL, GREATER, GEQUAL enable anarbitrarily-specified left hand side value to be compared to anarbitrarily-specified right hand side value. Thus, example opcodesALWAYS, NEVER, EQUAL, NOTEQUAL, LESS, LEQUAL, GREATER, GEQUAL may beused for RayOp tests that depend on some geometric aspects of either theray or the tested node, and moreover may be used for RayOp tests thatare independent of any geometric properties of either or both the rayand the tested node. Thus, in example non-limiting embodiments,“FLT_TMIN_LESS”, “FLT_TMIN_GEQUAL” and “FLT_TMAX_LESS”, and“FLT_TMAX_GEQUAL” RayOp tests actually evaluate the expressionsbbox.tmin<A*rval+B, bbox.tmin>=A*rval+B, bbox.tmax<A*rval+B,bbox.tmax>=A*rval+B, respectively. In one particular non-limitingembodiment, rval is an FP0.6.2 value and A and B are FP1.5.10 values forthese operations. Moreover, in some non-limiting example embodiments,since the FLT_TMIN and FLT_TMAX tests operate on the bounding box tminand bounding box tmax values which may be geometric values computed inthe intersection test, these RayOps may be used for geometriclevel-of-detail (e.g., where A corresponds to the cosine of the angle ofthe cone subtends the image plane pixel and B corresponds to theaccumulated length of the previous bounces of the ray and rvalcorresponds to the max_length of the bounding box). In some embodiments,the opcodes (e.g., FLT_TMIN_LESS, FLT_TMAX_LESS) provides for comparinga value computed during the ray/acceleration data structure intersectiontest scaled by one geometric attribute associated with the ray andbiased by another geometric attribute associated with the ray to atleast one geometric parameter associated with the at least one node.

At step 1710, one or more mode flags (sometimes also referred to as“mode specifiers”) corresponding to the RayOp test result areidentified. Each mode flag may be specified, for example, in apredetermined bit position in a ray data structure, and may include anynumber of bits. Each mode flag maps a result of the RayOp test or acombination of the result of the RayOp test and a node type of thetested node, to a particular action to be taken by the TTU. In someembodiments, the mode flags are separately specified with the ray forray-complet testing and ray-primitive testing respectively. Thus, inresponse to completing the RayOp test at step 1710, the applicable modeflag(s) may be found in the RCT mode flags specified for the ray.

In the above example, since the RayOp test failed, the applicable modeflag(s) include the “ch_f mode flag”. As described above, “ch_f”represents that the RayOp test failed for intersected child of typecomplet.

At step 1712, an action to be performed based on the identified modeflag(s) and/or ray flags is identified, and performed.

RCT mode flags express for each complet child type (e.g., complets,instance leaf nodes, item range leaf nodes, primitive range leaf nodes)how the TTU is to handle ray intersections with child-bounding-volumesfor child nodes of that type for those rays that pass or fail the RayOptest. Example RCT mode flags include “In_f”, “In_p”, “Ir_f”, “Ir_p”,“pr_f”, “pr_p”, “ch_f”, and “ch_p”.

The mode flag “In_f” (“modeInstanceNodeFail”) specifies an action to beperformed when the RayOp test fails for intersected child of typeinstance node (“InstanceNode”). The supported actions may includeprocessing in TTU, culling (e.g., suppress push of instance node ontotraversal stack), return as node reference, or return to SM.

The mode flag “In_p” (“modeItemRangePass”) specifies an action to beperformed upon the RayOp test passing for an intersected child of typeinstance node. The supported actions may include processing in TTU,culling (e.g., suppress push of instance node onto traversal stack),return as node reference, or return to SM.

The mode flag “Ir_f” (“modeItemRangeFail”) specifies an action to beperformed upon the RayOp test failing for an intersected child of typeitem range (“ItemRange”). The supported actions may include returning toSM (e.g., push item range hit into the result queue), culling (e.g.,suppress storage of item range hit in the result queue), or return asnode reference.

The mode flag “Ir_p” (“modeItemRangePass”) specifies an action to beperformed upon the RayOp test passing for an intersected child of typeitem range. The supported actions may include return to SM (e.g., pushitem range hit into the result queue), cull (e.g., suppress storage ofitem range hit in the result queue), or return as node reference.

The mode flag “pr_f” (“modePrimitiveRangeFail”) specifies an action tobe performed upon the RayOp test failing for an intersected child oftype primitive range (“PrimitiveRange”). The supported actions mayinclude processing in TTU (e.g., push entry onto traversal stack), cull(e.g., suppress push of triangle range stack entry onto traversalstack), return as node reference, or return to SM.

The mode flag “pr_p” (“modePrimitiveRangePass”) specifies an action tobe performed upon the RayOp test passing for intersected child of typeprimitive range. The supported actions may include processing in TTU(e.g., push entry onto traversal stack), cull (e.g., suppress push ofprimitive range stack entry onto traversal stack), return as nodereference, or return to SM.

The mode flag “ch_f” (“modeCompletFail”) specifies an action to beperformed when the RayOp test fails for an intersected child of typecomplet (“complet”). The supported actions may include traversing inTTU, cull, or return as node reference.

The mode flag “ch_p” (“modeCompletPass”) specify an action to beperformed when the RayOp test passes for an intersected child of typecomplet. The supported actions may include traversing in TTU, cull, orreturn as node reference.

In some embodiments, in addition to the mode flag(s) selected inaccordance with the RayOp test result, the selected action may beperformed in a manner consistent with one or more ray flags specified inthe ray data. The ray flags, such as ray flags 1514, may specifybehavior independent of any particular intersection.

In example embodiments, the ray flags may specify an order of traversalfor the bounding volumes, whether or not to pop the stack on return,whether or not to report node references to the SM when the ray's tmintmax interval starts inside the node's bounding box, whether or not toreturn at the first hit of an intersection, front-facing settings, cullsettings and the like.

The ray flags for traversal order may specify any one of: traversal inorder of parametric distance along the ray, traversal in memory order ofthe bounding volumes and/or primitives, decreasing x coordinate,increasing x coordinate, decreasing y coordinate, increasing ycoordinate, decreasing z coordinate, and increasing z coordinate, etc.More specifically, the traversal order dictates the order that stackentries get pushed onto the traversal stack when complet child boundingvolumes are intersected by the ray. In particular, when a node isintersected, the traversal order specified by the ray flags may be usedby the TTU to determine in which order the child nodes of theintersected node are to be pushed into the traversal stack. It is usefulfor example for tracing shadow rays that are set to return on the firsthit found and not specifically the nearest hit, where it is desirablefor such rays to first test against larger primitives (and thus morelikely to be hit). If the BVH is built in such a manner that the memoryorder of leaf node children is largest-first, then it is desirable tochoose memory order over t-order for such rays because it is more likelyto return quicker to the SM and t-ordering is immaterial for such rays.

One may desired to change traversal order (t-order) for any of severalreasons. For example, when trying to find the closest triangle, onewould typically want to use —order so that those primitives that mightcome earlier in parametric length are tested first. If those primitivesare intersected, then primitives and complets farther along the ray maynot need to be tested. When trying to find any intersection (e.g., totest if a point is in shadow from a light), then one may not care aboutwhich specific primitives are intersected and may want to test theprimitives that are most likely to be intersected first. In that case,the BVH builder may put the largest triangles earlier in the tree suchthat memory order will find them first.

The x/y/z ordering of traversal each may be used to approximatet-ordering in the case when t-ordering may not be consistent.Specifically, the t-intersection for a beam traversal and a raytraversal may not be consistent because the queries are differentshapes. (e.g., they may be similar, but not identical). The x/y/zordering, however, are each based on the bounding volume positionsalone, and are consistent. If the processing requires something likesharing the stack between a beam and a ray, then one may use theconsistent x/y/z ordering to get performance close to t-order.

The ray flags for indicating whether to pop the traversal stack onreturn (e.g., “noPopOnReturn”), may specify whether the stack is to bepopped, and/or whether to return the result of the traversal withoutpopping the stack. Returning the result of the traversal without poppingthe traversal stack may enable the SM to rerun the same traversal ormodify the stack before starting a new traversal.

The ray flags controlling the reporting of hits (e.g., “reportOnEnter”flag) may specify that the TTU is to only report a child hit if AABBintersection point t is greater than or equal to the ray's tmin, and tocull (and/or not report to the SM) otherwise. This flag enables abounding volume to not be reported to the SM even if it is intersected,if that intersection point (upon the ray's entry to the bounding volume)occurs before the ray's specified area of interest. One example use ofthis flag is for ray marching where after finding an intersection, thetmin is advanced to be the start of that intersection. On relaunch onemay want to find the next intersection, but typically would not want toreport again the intersection that was just returned. By setting thereportOnEnter flag, returning the intersection again to the SM can beavoided because a relaunched ray does not enter the volume, but ratherstarts inside of it.

The ray flags controlling whether to terminate upon the first hit (e.g.,“terminateOnHit”) specifies whether the TTU is to return at the firsthit for the ray found during traversal, or to keep on traversing untilit can return the parametrically nearest hit found.

The ray flags(s) that indicate what triangles are to be considered frontfacing (e.g., “facingfrontFaceCW”) may be used to specify certaintreatment of intersected leafs. For example, these flags may specifytreatment of counterclockwise winding triangles as front facing, ortreatment of clockwise winding triangles as front facing assumingright-handed coordinate system.

Ray flags controlling culling of intersected primitives (e.g.“cullMode”) may be specified to indicate no culling, cull back-facingprimitives, cull front facing primitives, or to disable culling andprimitive edge testing.

The traversal logic (e.g., traversal logic block 1012) performs theaction enumerated by the appropriate mode flag(s) based on the result ofthe RayOp test (or the inverse of the result of the RayOp test, if thechild invert flag is set). In the above example, since the ch_f modeflag indicates that the child bounding volume is to be culled when theyRayOp test fails, then the traversal logic will not push a stack entryonto the ray's traversal stack for this child bounding volume eventhough the ray may intersect the child's bounding volume and the defaultbehavior for intersected child bounding volumes is for the child to bepushed into the traversal stack. Note that ray could have, instead ofspecifying a value for ch_f mode flag indicating that the child is to beculled if the RayOp test fails, indicated alternatively that the childis to be traversed in the TTU, or be returned as a node reference.

The action by the traversal logic may be performed, at step 1714 in amanner consistent with ray flags of the ray. For example, where the rayflags indicate a particular traversal order, the child bounding volumesselected for traversal in accordance with the RayOp test may be pushedto the traversal stack in a manner consistent with the traversal orderspecified by the corresponding ray flag(s).

Since in step 1712, the child nodes that are selected according to theRayOp test are pushed on to the traversal stack, the selected childnodes are traversed in the order that they are popped from the traversalstack.

Steps 1704-1714 may be repeated for each child of the intersectedbounding volume. When each of the child nodes, or at least each of thechild nodes that are themselves found to intersect with the ray have hada RayOp performed, the parent bounding volume has completed itstraversal step. That is, in the case where a complet includes only aroot bounding volume and its child bounding volumes, the traversal ofthat complet has completed. More generally, as when the complet includesa root and more than one level of nodes, the traversal of the complet iscomplete when all the leaf nodes of the complet, or at least all thosethat have not been culled, have been subjected to the ray-boundingvolume intersection test and/or the RayOp test.

The above description of process 1700 applied to, at step 1702,determining that the ray intersects the node and the node does not havethe FRA flag set. If instead, at step 1702 it is determined that the rayintersects the node and that the FRA flag is set at the node, then theray's programmable ray operation is overridden to the “ALWAYS” rayoperation which effectively ignores the ray's programmable ray operationwith respect to the child nodes and proceeds to treat the child nodes asnodes to be traversed. This override remains in effect only for thatnode/complet. When the FRA flag being set is implemented by overridingthe ray's ray operation to “ALWAYS”, the per child node invert bit maystill apply to allow the complet to still use both the pass and failmode flags, although the decision is static instead of dynamic.

Example Image Generation Pipeline Including Ray Tracing

While the above disclosure is framed in the specific context of computergraphics and visualization, ray tracing and the disclosed TTU could beused for a variety of applications beyond graphics and visualization.Non-limiting examples include sound propagation for realistic soundsynthesis, simulation of sonar systems, design of optical elements andsystems, particle transport simulation (e.g., for medical physics orexperimental high-energy physics), general wave propagation simulation,comparison to LIDAR data for purposes e.g., of robot or vehiclelocalization, and others. OptiX™ has already been used for some of theseapplication areas in the past.

For example, the ray tracing and other capabilities described above canbe used in a variety of ways. For example, in addition to being used torender a scene using ray tracing, they may be implemented in combinationwith scan conversion techniques such as in the context of scanconverting geometric building blocks (i.e., polygon primitives such astriangles) of a 3D model for generating image for display (e.g., ondisplay 750 illustrated in FIG. 7 ).

Meanwhile, however, the technology herein provides advantages when usedto produce images for virtual reality, augmented reality, mixed reality,video games, motion and still picture generation, and othervisualization applications. FIG. 18 illustrates an example flowchart forprocessing primitives to provide image pixel values of an image, inaccordance with an embodiment. As FIG. 18 shows, an image of a 3D modelmay be generated in response to receiving a user input (Step 1852). Theuser input may be a request to display an image or image sequence, suchas an input operation performed during interaction with an application(e.g., a game application). In response to the user input, the systemperforms scan conversion and rasterization of 3D model geometricprimitives of a scene using conventional GPU 3D graphics pipeline (Step1854). The scan conversion and rasterization of geometric primitives mayinclude for example processing primitives of the 3D model to determineimage pixel values using conventional techniques such as lighting,transforms, texture mapping, rasterization and the like as is well knownto those skilled in the art. The generated pixel data may be written toa frame buffer.

In step 1856, one or more rays may be traced from one or more points onthe rasterized primitives using TTU hardware acceleration. The rays maybe traced in accordance with the one or more ray-tracing capabilitiesdisclosed in this application. Based on the results of the ray tracing,the pixel values stored in the buffer may be modified (Step 1858).Modifying the pixel values may in some applications for example improvethe image quality by, for example, applying more realistic reflectionsand/or shadows. An image is displayed (Step 1860) using the modifiedpixel values stored in the buffer.

In one example, scan conversion and rasterization of geometricprimitives may be implemented using the processing system describedabove, and ray tracing may be implemented by the SM 732 using the TTU738 architecture described in relation to FIG. 10 , to add furthervisualization features (e.g., specular reflection, shadows, etc.). FIG.18 is just a non-limiting example—the SM's 732 could employ thedescribed TTU by itself without texture processing or other conventional3D graphics processing to produce images, or the SM's could employtexture processing and other conventional 3D graphics processing withoutthe described TTU to produce images. The SM's can also implement anydesired image generation or other functionality in software depending onthe application to provide any desired programmable functionality thatis not bound to the hardware acceleration features provided by texturemapping hardware, tree traversal hardware or other graphics pipelinehardware.

The TTU 738 in some embodiments is stateless, meaning that noarchitectural state is maintained in the TTU between queries. At thesame time, it is often useful for software running on the SM 732 torequest continuation of a previous query, which implies that relevantstate should be written to registers by the TTU 738 and then passed backto the TTU in registers (often in-place) to continue. This state maytake the form of a traversal stack that tracks progress in the traversalof the BVH.

A small number of stack initializers may also be provided for beginninga new query of a given type, for example:

-   -   Traversal starting from a complet    -   Intersection of a ray with a range of triangles    -   Intersection of a ray with a range of triangles, followed by        traversal starting from a complet    -   Vertex fetch from a triangle buffer for a given triangle    -   Optional support for instance transforms in front of the        “traversal starting from a complet” and “intersection of a ray        with a range of triangles”.

Vertex fetch is a simple query that may be specified with request datathat consists of a stack initializer and nothing else. Other query typesmay require the specification of a ray or beam, along with the stack orstack initializer and various ray flags describing details of the query.A ray is given by its three-coordinate origin, three-coordinatedirection, and minimum and maximum values for the t-parameter along theray. A beam is additionally given by a second origin and direction.

Various ray flags can be used to control various aspects of traversalbehavior, back-face culling, and handling of the various child nodetypes, subject to a pass/fail status of an optional rayOp test. RayOpsadd considerable flexibility to the capabilities of the TTU. In someexample embodiments, the RayOps portion introduces two Ray Flag versionscan be dynamically selected based on a specified operation on dataconveyed with the ray and data stored in the complet. To explore suchflags, it's first helpful to understand the different types of childnodes allowed within a BVH, as well as the various hit types that theTTU 738 can return to the SM. Example node types are:

-   -   A child complet (i.e., an internal node)        By default, the TTU 738 continues traversal by descending into        child complets.    -   A triangle range, corresponding to a contiguous set of triangles        within a triangle buffer    -   (1) By default, triangle ranges encountered by a ray are handled        natively by the TTU 738 by testing the triangles for        intersection and shortening the ray accordingly. If traversal        completes and a triangle was hit, default behavior is for the        triangle ID to be returned to SM 732, along with the t-value and        barycentric coordinates of the intersection. This is the        “Triangle” hit type.    -   (2) By default, intersected triangles with the alpha bit set are        returned to SM 1840 even if traversal has not completed. The        returned traversal stack contains the state required to continue        traversal if software determines that the triangle was in fact        transparent.    -   (3) Triangle intersection in some embodiments is not supported        for beams, so encountered triangle ranges are by default        returned to SM 1840 as a “TriRange” hit type, which includes a        pointer to the first triangle block overlapping the range,        parameters specifying the range, and the t-value of the        intersection with the leaf bounding box.    -   An item range, consisting of an index (derived from a        user-provided “item range base” stored in the complet) and a        count of items.

By default, item ranges are returned to SM 1840 as an “ItemRange” hittype, consisting of for example an index, a count, and the t-value ofthe intersection with the leaf bounding box.

-   -   An instance node.

The TTU 738 in some embodiments can handle two levels of instancingnatively by transforming the ray into the coordinate systems of twoinstanced BVHs. Additional levels of instancing (or every other level ofinstancing, depending on strategy) may be handled in software (or inother embodiments, the TTU 738 hardware can handle three or more levelsof instancing). The “InstanceNode” hit type is provided for thispurpose, consisting of a pointer to the instance node and the tvalue ofthe intersection with the leaf bounding box. In other implementations,the hardware can handle two, three or more levels of instancing. Aninstance node may also be configured with an instance mask thatindicates the node's participation none, one or more than one groups ofgeometry that is selectable on a per-ray basis with an instanceinclusion mask included in the ray. A valid flag may also be availableto indicate whether the instance mask is valid or invalid.

In addition to the node-specific hit types, a generic “NodeRef” hit typeis provided that consists of a pointer to the parent complet itself, aswell as an ID indicating which child was intersected and the t-value ofthe intersection with the bounding box of that child.

An “Error” hit type may be provided for cases where the query or BVH wasimproperly formed or if traversal encountered issues during traversal.

A “None” hit type may be provided for the case where the ray or beammisses all geometry in the scene.

How the TTU handles each of the four possible node types is determinedby a set of node-specific mode flags set as part of the query for agiven ray. The “default” behavior mentioned above corresponds to thecase where the mode flags are set to all zeroes.

Alternative values for the flags allow for culling all nodes of a giventype, returning nodes of a given type to SM as a NodeRef hit type, orreturning triangle ranges or instance nodes to SM using theircorresponding hit types, rather than processing them natively within theTTU 738.

Additional mode flags may be provided for control handling of alphatriangles.

Images generated applying one or more of the techniques disclosed hereinmay be displayed on a monitor or other display device. In someembodiments, the display device may be coupled directly to the system orprocessor generating or rendering the images. In other embodiments, thedisplay device may be coupled indirectly to the system or processor suchas via a network. Examples of such networks include the Internet, mobiletelecommunications networks, a WIFI network, as well as any other wiredand/or wireless networking system. When the display device is indirectlycoupled, the images generated by the system or processor may be streamedover the network to the display device. Such streaming allows, forexample, video games or other applications, which render images, to beexecuted on a server or in a data center and the rendered images to betransmitted and displayed on one or more user devices (such as acomputer, video game console, smartphone, other mobile device, etc.)that are physically separate from the server or data center. Hence, thetechniques disclosed herein can be applied to enhance the images thatare streamed and to enhance services that stream images such as NVIDIAGeForce Now (GFN), Google Stadia, and the like.

Furthermore, images generated applying one or more of the techniquesdisclosed herein may be used to train, test, or certify deep neuralnetworks (DNNs) used to recognize objects and environments in the realworld. Such images may include scenes of roadways, factories, buildings,urban settings, rural settings, humans, animals, and any other physicalobject or real-world setting. Such images may be used to train, test, orcertify DNNs that are employed in machines or robots to manipulate,handle, or modify physical objects in the real world. Furthermore, suchimages may be used to train, test, or certify DNNs that are employed inautonomous vehicles to navigate and move the vehicles through the realworld. Additionally, images generated applying one or more of thetechniques disclosed herein may be used to convey information to usersof such machines, robots, and vehicles.

All patents & publications cited above are incorporated by reference asif expressly set forth. While the invention has been described inconnection with what is presently considered to be the most practicaland preferred embodiments, it is to be understood that the invention isnot to be limited to the disclosed embodiments, but on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims.

1. A ray tracing acceleration hardware device, comprising: memoryconfigured to store at least portions of an acceleration data structure;ray storage configured to store an opcode from a ray received from aprocessor; traversal circuitry configured to traverse the accelerationdata structure according to the ray, the traversing including, at a nodeof the acceleration data structure, (a) performing an operationspecified by the opcode if a flag is in a first state, and (b) ignoringthe operation if the flag is in a second state; and intersectiondetection circuitry configured to detect one or more intersections ofthe ray with one or more nodes of the acceleration structure during thetraversing and return information of the detected intersection to theprocessor.